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PAGE # 1SOLUTION OF TRIANGLE
SOLUTION OF TRIANGLE GENERAL NOTATION :
1. In a triangle ABC angles at vertices are usually denoted by A, B, C
sides opposite to these vertices are denoted by a, b, c respectively.
2. Circumradius is denoted by R
3. Inradius is denoted by ‘r’.
4. Ex-radii are denoted by 1 2 3r , r , r opposite to vertices A, B, C respectively..
5. 2s = a + b + c2s – 2c = a + b – ca + b – c = 2(s – c)a + c – b = 2(s – b)
PAGE # 2SOLUTION OF TRIANGLE
2 2a b c = a b c a b c
= 4s(s – c)
2 2a b c a b c a b c
= 4 s a s b
s a , s b , s c will be always +ve.
6. ’’ or S is used for area of triangle.
7.1
pa2
2pa
perpedicular = 2 Ar. ofopp. side
.
8. 2 2 2 2AB AC 2 AD BD
2
2 2 2 ac b 2 AD4
2 2 2
22 b c a
2AD2
length of median, AD = 2 2 22 b c a
2
.
2. In a triangle the angle opp. to greatest side will be greatest.
3. In a centroid and incenter will always lie within it whereas orthocentre and circumcentre may lie outsidethe .
4. The ratio of the area of the triangles made on same base (altitudes) will be equals to ratio of altitudes (base)
Ar ABD BDAr ADC DC
PAGE # 3SOLUTION OF TRIANGLE
2
1
Ar ABC pAr DBC p
4. In aABC(i) sin A, sin B, sin C > 0(ii) (a) sin(A + B) = sin C(b) cos(A + B) = –cos C(c) tan(A + B) = –tan C
(iii) (a) A B Csin cos2 2 2
(b) A B Ccos sin2 2 2
(c) A B Ctan cot2 2 2
5. # Sine Rule :
In ABC
a b c 2RsinA sinB sinC
In ABD & ACD :PROOF : AD = AD
c sinB bsinC
In ABD ACD
b a
sinB sinA O is circumcentre
In BOM BM = a2
asinA2
a = 2R sin AA
we have a b c 2R
sinA sinB sinC .
PAGE # 4SOLUTION OF TRIANGLE
PATTERN IDENTIFICATION :
(i) a sin AA (i.e. a and sin(A) are mutually convertible)
(ii) 2 2a b sin A B .sin A B
sinC.sin A B
PROOF : 2 2a b = 2 2 24R sin A sin B
= 24R sin A B .sin A B
= 24R sin A B .sinC .
(iii) a b sinA sinB
c sinC
a b 2R sinA sinB
= A B A B4Rsin cos
2 2
= C A B4Rsin sin2 2
(iv) acosB sin2A
(v) a tanA
cos A .
6. To find a side indentify the in which it lies preferably right angle or the triangle one side of which and two of theangles are known.
7. If in the ques. two of the angles say A & B and one of the corresponding sides say a is given this may impliessine rute.
NAPIER’S ANALOGY :
1.b c sinB sinCb c sinB sinC
=
B C B C2sin cos2 2
B C B C2sin cos2 2
= B C B Ctan cot
2 2
= B C Atan .tan
2 2
B C b c Atan cot2 b c 2
PAGE # 5SOLUTION OF TRIANGLEwhich gives the result :
B C b c Atan tan2 b c 2
Similarly A C a c Btan tan
2 a c 2 &
A Btan2
= a b C
tana b 2
..
NOTE : If trigonometric ratio of difference of two of the angles (say A – B) and the corresponding sides (a & b) are givenin the ques this may imply Napier ’s analogy.
PROJECTION FORMULA :
BC = BD + DC
a c cosB bcosC
a c cosB bcosC
Similarly b = acosC c cos A
& c = acosB bcosA .
COSINE RULE :
1. In ABD (refer above figure)2 2 2AB AD AD
22 2 2C a bcosC b sin C
= 2 2 2 2 2a b cos C 2abcosC b sin C 2 2 2C a b 2abcosC 2 2 2b a c 2ac cosB 2 2 2a b c 2bc cos A
2 2 2a b c
cos C2ab
Similarly 2 2 2b c a
cosA2bc
2 2 2a c bcosB
2ac
.
GENERAL NOTE :
(i) for three quantities a, b, c.
(i) a b 0 (ii) a b 2 a (iii) b c a 0
PAGE # 6SOLUTION OF TRIANGLE
(iv) a b c a (v) ab a b a b .
NOTE:
1. If two of the sides (say a & b) and the third angle (C) is given in the ques. then this may imply application ofcosine rule.
2. bc cos A = 2 2 21 b c a2
3. cot A
= cot Asin A
= 2 2 2b c a2bc sinA
= 2 2 2b c a
4
= 2 2 21a c a
4
= 2a
4 =
= 2 2 2a b c
4
.
4. If ABCD is cyclic quadrilateral :2 2BD BD
2 2 2 2a d 2adcos A b c 2bccosA
2 2 2 22cos A ad bc a b b c
2 2 2 2a d b ccosA
2 ad bc
2 2 2 2a b c dcosB
2 ab cd
cos C = 2 2 2 2b c a d
2 bc ad
2 ab cd ac bdBD
ad bc
Similarly
2 ac bd ad bc
ACab cd
PAGE # 7SOLUTION OF TRIANGLE
22 2BD .AC ac bd
BD.AC = ac + bdBD.AC = AB.CD + AD.BC(Ptolemy’s Theorem)
NOTE : If A = 3
then cos A cos3
= 2 2 2b c a
2bc
.
2 2 21 b c a2 2bc
2 2 2a b c bc .
Q.1 (i) If a = 5, b = 4 & cos (A – B) = 3132
find C.
Q.2 P.T. b c cos A a .
Q.3 P.T. a bcos c ccos B = 2 2b c .
Q.4 If a 1 3, b 2 & c 60 , Find other sides & angles.
Q.5 If C2
, a = 3, b = 4 and
D is on A B such that BCD6
Find CD.
6. If , , are legnth of altitudes of ABC.
T.P.T. 2
cot A1
.
7. P.T.
A Aasin B b c sin2 2
.
8. If b c a c a b11 12 13
T.P.T. cos A cosB cosC
7 12 25 .
9. P.T. 2 2
2b c .sin 2A 0
a
.
10. P.T. a
2asec A tanBtanCcosB cosC
.
PAGE # 8SOLUTION OF TRIANGLE
1.
2 1 cos A BA Btan2 1 cos A B
1 cos A BA Btan
2 1 cos A B
= 1 31/ 321 31/32
= 1
3 7.
Using Napier’s
A B a b ctan cot2 a b 2
= 1 1 c
cot9 23 7
c 7
tan2 3
1 7 /9cosc1 7 / 9
1
cosc8
.
2 2 2c a b 2abcosc
= 125 16 2 5 48
= 36
c = 6.
Q.2 L.H.S. = bcos A ccos A + acos B ccos B + acos C bcos C
= c + b + a = a .
Q.3 abcosC ac cosB
= 2 2 2 2 2 2a b c a c b
2 2
= 2 2
2 22 b c
b c2
.
Q.4 cosine Rule :2 2 2c a b 2abcosc
= 2 11 3 4 2.2. 1 3
2
PAGE # 9SOLUTION OF TRIANGLE
= 1 3 3 1 2 4
= 3 1 3 1 4 = 6
= c = 6 .
Q.5 2 BCD :
CD BCsinB
sin B6
CD = 3sinB
sin B6
=
435
1 3 3 42 5 2 5
.
Q.62a
2b
2c
2
2 2
a14
= 2 2 2a b c 1
4
= 1 cot A .
Q.7 consider
b c sinB sinCa sinA
=
B C B C2sin cos2 2
A A2sin cos2 2
=
B Ccos2Asin2
(As A B Ccos sin2 2
)
PAGE # 10SOLUTION OF TRIANGLE
=
C Bcos2Asin2
=
A B Bcos2Asin2
A Ab c sin acos B2 2 2
= Aa sin B2
.
Q.8b c a c a b11 12 13
= 2 a b c
36
(Using C & D)
c b a5 6 7
c 5
b 6 a = 7.Apply cosine rule
2 2 2b c acosA
2bc
= 36 25 49
60
etc.
Q.92 2
2b c sin2A
a
= 2 2
2b c 2sinA cosA
a
= 2 2b c
cos AR.a
= 2 2 2 2 2b c b c a
2Rabc
= 2 4 2 2 21 b c a b c2Rabc
= 0.
10.a
cosBcosC
= 1
acos AcosA cosBcosC
PAGE # 11SOLUTION OF TRIANGLE
= 1
2RsinA cos AcosA cosBcosC
= R4sinA sinBsinC
cosA cosBcosC (using sin 2A 4 sin A )
= 2asec A tanB tanC
HALF ANGLE FORMULAE :
2 2 2b c a
cos A2bc
2 2 2
2 A b c a2cos 12 2bc
2 2 2
2 A b c a2cos 12 2bc
=
2 2 2b c a 2bc2bc
= 2 2b c a
2bc
= 2s.2 s a
2bc
s s aAcos
2 bc
Similarly s s bBcos
2 ac ,
s s cCcos2 ab
Again 2 Acos A 1 2sin2
= 2 2 2b c a
2bc
2 2 2
2 A b c a2sin 12 2bc
= 22a b c
2bc
s b s cAsin
2 bc
Similarly s a s cBsin
2 ac
also,
s b s cA
tan2 s s a
,
s a s cBtan
2 s s b
.
AREA OF TRIANGLE :
1.1 ht base2
= 1 c sinB a2
PAGE # 12SOLUTION OF TRIANGLE
= 1 ac sinB2
= 1 bc sinA2
[using a sin B = b sin A]
= 12RsinB 2R sinC sinA
2
= C
2
A
2R sinA .
Also
1bc sinA2
= 1 A Ab.c.2sin cos2 2 2
= s b s c s s a
b.c. .bc bc
= S s a s b s c
1absinC
2
= 22R sinA= S s a s b s c .
*
s b s cA
tan2 S s a
= s b s c
= s s a
.
*
S s bB
cot2 s a s c
= s a s c
= s s b
.
*2
sin Abc
.
* If ques. contains half s and sides, then use above formula for manipulation.
COT - m-n THEOREM :
If base BC is divided by pt. D in ratio m : n, then
PAGE # 13SOLUTION OF TRIANGLE
(1) (m + n) cot = m cot – n cot (2) (m + n) cot = n cot B – m cot C.Proof : in ABD :
AD BD
sin sin
...(1)
In ACD :
AD DC
sinsin
...(2)
(1) (2)
sin BD sinsin DC sin
sin m sinsin n sin
nsin sin cos cos sin msin sin cos cos sin
Dividing by sin .sin .sin , we get
(m + n) cot = m cot – n cot
* If in question, one of the side is bisected/trisected etc. then this may imply application of cot m – n theorem.
Q.1 P.T.
Acot2
cot A
=
2
2
a
a
.
Q.2 If 22a b c , then find tan A.
Q.3 Let C3
, A 75 , If D is on ACAC
Ar. (BAD) = 3 Ar. (BCD). Find ABD.
Q.4 If median AD AB, in a , P.T. tan A + 2 tan B = 0.
Q.5 If medians AD & BE intersects at 2
, then P.T..
2 2 2a b 5c .
Q.6 Find ‘A’, if (a + b + c) (b + c – a) = 3bc.
Q.7 If A3
, T.P.T. 2 2 24AD b bc c , where AD is median.
Q.8 If A, B, C are in A.P., and if 2 23c 2b , then find angles of .
PAGE # 14SOLUTION OF TRIANGLE
Q.9 In a , P.T. A B 2c1 tan tan2 2 a b c
.
Q.10 If median AD is 'r to AC, T.P.T.s
2 22 c acosA cosC
3ac
.
Ans.1Acot2
=
s s a = 1 s s a
= 213s s a
= 2a b c4
also 2a
cot A4
Acot2
cot A
=
22
a
a .
Alt : A
cot2 =
Acot
2 = s s a
= 3s s a s b s c
=
2s
.
2. a b c a b c
= 4 s b s c
s b s c
= 4 A 1tan2 4
s b s cAtan
2 s s a
= s b s c
2
A2tan2tan AA1 tan2
=
12411
16
=
815
.
3. cot m – n th :
3 1 75 1.cot 75 3 cot60
PAGE # 15SOLUTION OF TRIANGLE
4.
11 1 cot B 1.cot A cot2 2 2 2
2tanB tan A
tanA 2 tanB 0 .
5. Dist. of A frm. centroid = AG = 2AD
3 =
2 2 22 b c a
3
2 2 2AE AG GE (where is mid pt. of AC)
2 2 22 2 b c ab
2 9
+
2 2 22 a c b
36
.
6. s s a 32s2 s a 3bc
bc 4
= 2 Acos2
= 34
A 3
cos2 2 A = A =
3
Alt :
2 2b c a 3bc
2 2 2b c a
1bc
2 2 2b c a 1
2bc 2
= cos A A3
.
7. A = 3
2 2 2b c bc a Also
PAGE # 16SOLUTION OF TRIANGLE
AD = 2 2 22 b c a
2
2 2 2 24AD 2 b c a
2 2 2 22 b c b c bc
= 2 2b c bc .
8. A, B, C A.P..
B3
2 23c 2b
c 2b 3 =
sinCsinB
2 3 2 1sinC sin .
3 3 2 3 2
C4
A = 3 4 .
9.A B1 tan tan2 2
=
s b s c s a s c
1s s a s s b
= s c
1c
= c 2cs a b c
.
10. 2 2 22
2 2 22 b c aa b AD b
2 4
2 2 2 2 2a 4b 2b 2c a
2 2 2a c 3b
2 2 2b c acosA
2bc
2 2 2b a ccosC
2ab
PAGE # 17SOLUTION OF TRIANGLE
= 2 2b 3b2ab
2 2 22 a c 4b
cos AcosC3.2bc 2ab
= 2 22 c a
3ac
.
Q.1 P.T.
2
2
B Ccos2
b c
+
2
2
B Csin2
b c
= 2
1a
.
Q.2 ABCD is a trapezium such that AB & DC are || & BC is 'r to then. If ADB = , BC = p & CD = q,
T.P.T. 2 2p q sin
ABbcos qsin
.
Q.3 If 2 ab c
tan sina b 2
,
T.P.T. c a b sec .
Q.4 P.T. 2 22 2 2c cc a b cos a b sin2 2
.
Q.5 P.T. a cosBcosC cosA = c cosAcosB cosC = b cos A .cos C cos B .
Q.6 2 2 2a cos2B b cos 2A 2abcos A B c .
Q.7 2 2Abccos s2
.
Q.8 2b c Acos 0a 2
Q.9A2 acos2
= a
× .
Q.10
Atan 12a b a c
.
Sol.1 L.H.S =
2 2
2 2
B C B Ccos sin2 2
b c b c
PAGE # 18SOLUTION OF TRIANGLE
=
2
22
B Ccos2
4R sin B sin C
+
2
22
B Csin2
4R sin B sin C
= 22 2
1 1 1B C B C4R 4sin 4cos
2 2
=
2 2
22 2
B C B Csin cos1 2 2
B C B C4R 4sin cos2 2
= 2 2
1 14R sin B C
= 2
1a
.
Sol.2 In BAD
AB BD
sin sin
AB =
sin .BD
sin .cos cos .sin
=
2 2
2 2 2 2
sin . p qq psin .cos
p q p q
=
2 2p q .sin
qsin pcos
.
Sol.3 2 2sec 1 tan
=
2
2
C4absin21
a b
=
2
2
a b 2ab 1 cos C
a b
=
2 2
2
a b 2abcos C
a b
=
2
2C
a b .
PAGE # 19SOLUTION OF TRIANGLE
Sol.4 2 22 2C Ca b .cos a b .sin2 2
= 2 2 2 2C Ca b cos sin2 2
– 2 2C C2ab cos sin
2 2
= 2 2a b 2abcos C = C2.
Sol.5 a cos B cos C cos A
= a cos B cos C cos B C
= a cos B .cos C cos B cos C sin B sin C
= a sin B sin C
= 2Rsin A sin B sin C
Using symt. all terms are equals to 2R sin A .
Sol.6 2 2a cos 2B b cos 2A 2abcos A B
= 2 2 2a cos B sin B + 2 2 2b cos A sin A + 2ab cos A cos B sin A sin B
= 2 2 2 2a cos B b cos A 2abcos A cos B – 2 2 2 2a sin B b sin A 2absin A sin B
= 2 2acos B bcos A asin B bsin A
= 2 2c 0 c .
Sol.7 2 Abc.cos2
= s s a
bc.bc = s s a = s(s) = s2.
Sol.8 2b c A.cosa 2
= s s ab c
.a bc
= sb c s a
abc
= ss b c a b c 0
abc .
PAGE # 20SOLUTION OF TRIANGLE
DIFFERENT CENTRES OF A TRIANGLE :
A. Circum centre :
(1) abcR4
(2) OM = Rcos A = Dist. of BC frm. c.c. Dist of side frm c.c. = R cos A, R cos B, R cos C.
B. Incentre :
(1) BD cDC b
.
(2) (a) BD = c
ab c
||ly DC = ab
b c(3) In BAD :
IA ABID BD
= c
acb c
= b c
a
.
(4) AIB = A B2 2
= c
2 2 .
(5) (i) In BAD :
AD BDAsinB sin2
AD = sinB C
sin A / 2 b c
[using a sin B = b sin A]
= 2bc A.cosb c 2
= length of r bisector..
similarly, 2ac BBE .cosa c 2
.
(ii) Ar ABC Ar IBC Ar (IAC) + Ar(IAB)
1 1 1ar br ar2 2 2
= r.s
r = s
PAGE # 21SOLUTION OF TRIANGLE
(iii) rs
= r . s as s a
= As a .tan2
= Bs b .tan2
= Cs c .tan2
.
(iv) A B C4Rsin .sin .sin2 2 2
s b s c s a s c
4R .bc ac
.
s a s bab
= 4R. s a s b s c
abc
= s s a4R.
abc s
= 21
. rs s
= Ar 4R sin2
.
(i) rs
= As a .tan2
= A4R sin2
(iv) In BPI
BI = Br cosec2
= A C4Rsin sin2 2
= Dis. of vertex B from incenter.
Dis. of vertices from incenter rcscA2
, rcsc
B2
and rcsc
C2
.
PAGE # 22SOLUTION OF TRIANGLEIn BPI
B rtan2 BP
BP =
r s bBtan2
= length of tangent from B to incircle. BP = BR = s – bCP = CQ = s – cAQ = AR = s – a
(vi) IAO = A B2 2
= A B C
B2 2 2 2
= B C
2
AI = B C4R sin sin2 2
AO = R.In AIO
2 2 2IO AI OA 2.OA.AI.cos IAO .
2 2 2 2 2B CIO R 16R sin .sin2 2
– B C B C2.R.4R sin sin .cos2 2 2
= 2 2 B C B C B CR 8R sin sin 2sin sin cos
2 2 2 2 2
= 2 2 AR 8R sin
2
IO = AR 1 8 sin2
= 2R 2Rr(i) R 2r
(ii) A 1sin2 8
Orhocentre :
(i) BD = c.cos B
(ii) DC = b.cos C
PAGE # 23SOLUTION OF TRIANGLE
(iii)
c cos BBDDC bcos C
=
tan Ctan B
(iv) BAD
BH = c.cos B .cosec C
= c .cos Bsin c
= 2Rcos B
HD = ccos B .cot C
=
cos Cc.cos B .
sin C
= 2Rcos B .cos C
Hence Dis. of verteces from orthocenter will be 2R cos (A)
2Rcos B , 2Rcos C and the distance of the sides from orthocenter will be 2R cos(B) cos(C),
2Rcos C cos A and 2Rcos A cos B
HAO B C AH = 2R cos(A)AO = R
HAC C2
AOM B
AOM B2
HAO C B2 2 = B – C.
2 2 2OH AO AH 2OA.AHcos B C
= 2 2 2R 4R cos A 2.R.2Rcos A cos B C
= 2R 1 4cos A cos A cos B C
= 2R 1 4cos A cos B C cos B C
= 2 2R 8R cos A = OH = R 1 8 cos A .
(iii) EX-CIRCLE :
1. (i) 1r s a
PAGE # 24SOLUTION OF TRIANGLE
(ii) 2r s b
(iii) 3r s c
2. 1A B Cr 4R sin .cos cos2 2 2
2A B Cr 4Rcos .sin .cos2 2 2
3. (a) 1Ar s tan2
(b) 2Br s tan2
(c) 3Cr s tan2
4. 21 2
A Br r s tan tan2 2
21 2r r s .
5.1
1 1r r
Proof:s a
= 1 s a
= 3s 2s
=
s 1r
.
6. 1 2 3r r rr
Proof: . .s a s b s c
= 2
2s ss a
.
Q.1 Prove that rcos A 1R
PAGE # 25SOLUTION OF TRIANGLE
Sol. cos A = A1 4 sin2
=
A4R sin21
R
= 1 + rR
.
Q.2 Prove that if 1 2 3r , r , r in H.P. then ABC are in A.P
Sol.1 2 3
1 1 1, ,r r r A.P..
s a s b s c, , .....r
A.P.
s – a, s – b, s – c A.P.–a, –b, –c A.P..a, b, c A.P..
Q.3 Let dis. of orthocenter from vertices is p, q, r then prove that
Proof: a.q.r abc p 2Rcos A
q 2Rcos B
r 2Rcos C
L.H.S. = aqr= 2Rsin A .2Rcos B .2Rcos C= 38R sin A .cos B .cos C= 38R . sinA abcALT :
1Ar ABC + 2 + 3
= 1 qr.sin B C2
= a
qr4R
1 2 3
abc aqr bpr cpr4R 4R 4R 4R
PAGE # 26SOLUTION OF TRIANGLE
= abc = aqr .
Q.1 If 1 2 3r r r r
P.T. is right angled.Q.2 If B / 2
P.T. r = a b c
2
.
Q.3 If altitudes from A, B, C are produced to meet circum centre, if length of produced parts is , , then prove that
centre, if length of produced parts is , , then prove that a2 tan A
.
Q.4 If , , are length of internal bisectors T.P.T. 1 A.cos
2 =
1a .
Q.51 1
ab 2Rr .
Q.61
b c 0r
.
Q.7 If , , are the distance of the vertex of a from the corresponding pts. of contect with the in-circle
T.P.T. 2r
.
A.1s a s b s c s
1 1 1 1s a s s b s c
s s as s a
= 2s a b cs b s c
2 2s as s b c s bc
s b c a bc
a b c b c a bc
2 2 2b c 2bc a 2bc
2 2 2b c a .
4. Br s b .tan2
= s b .tan4 = s – b
PAGE # 27SOLUTION OF TRIANGLE
= 2 s b
2
= a c b
2
.
5.1
ab = c
abc = 1
. cabc
= 2s
abc =
4 s 1. .
abc 2
=
1 1 1.
R r 2 .
b c
s a
= b c s a
= 1 b c b c = 0.
6.1
b cr =
b c. s a
= s b c – 1 a b c
= 0.
7. s a
s b
s c s
=
=
s a
s
= 2
s. s a
s
= 2
2rs
2r
.
4.2bc A.cosb c 2
Acosb c 1 1 122bc 2 b c
= Acos2
=
1 1 12 b c
PAGE # 28SOLUTION OF TRIANGLE
= 1 1 12 b c
= 1a .
3. As BPA = BCA
BPA c Also,
In BDP
BDcot C
=
cos Cc.cos B .
sin C
= 2Rcos B cos C
a a
2Rcos B cos C
=
sin A
cos B .cos C
=
sin B CC B .C C
= tan B tan C
= q = t B t C = 2 t A .
Q. If 1 2 3p , p , p are length of 'r from vertices to sides then P.T..
(i) 1
1 1p r
(ii) 1
cos A 1p R
(iii) 1bpc
+ 2cpa
+ 3cpb
= 2 2 2a b c
2R
.
Q.2 P.T. pro. IA = abcA B Ctan tan tan2 2 2
.
Q.3 If x, y, z are respectively distance of the vertex from its orthocentre then prove that
x 2 R r .
Q.4 If x, y, z respectively are the 'r from the circum center to the sides of the triangle ABC then P.T. a abcx 4xyz .
Q.5 (i) P.T. in a triangle 2 Atan 1
2 .
PAGE # 29SOLUTION OF TRIANGLE
(ii)
2 Aacos324a
.
(iii) 2s
3 3
(iv)
2 Aasin124a
.
Ans.1 12
pa
1
1p =
a2 =
a
2 =
2s2
= 1r
.
(iii)
1
cos A
p
= cos A .a
2
= R sin 2A2 = R .4 sin A
2
= 22R sin A
R .
(iii) 1b 2bpc a
2
1b .2
bpabc
1pbc =
2abc
. 2p = 21 a2R .
A.2. T.P. IA = Aa. tan2
L.H.S. = B C4Rsin sin2 2
= 3 A B C A B C64R s s s .s s s
2 2 2 2 2 2
= 3 s A .s B .s C A B C64R .t . .
8 2 2 2 = Aabc tan
2
.
Q.3 x 2Rcos A
PAGE # 30SOLUTION OF TRIANGLE
2Rsin Aa tan A
x 2Rcos A
ax = tan A = tan A =
ax
x = 2Rcos A
= A2R 1 4 sin2
= A2 R 4R sin2
= 2 R r .
Q.4 x = R cos A
2Rsin Aax Rcos A = 2 tan(A)
a2 tan A
x = 2 tan A
= a22x
=
1 a4 x
.
Q.5 Apply A.M. = G.M.
on 2 Atan
2 ,
2 Btan2
2 2A B A Btan tan tan .tan
2 2 2 2
similarly
adding 2 Atan 1
2 (As
A Btan tan 12 2
)
(ii) 2 Aacos
2a b c
1 cos Aa
2
a
= a acos A
2 a
PAGE # 31SOLUTION OF TRIANGLE
=
R sin 2A12 4R sin A
=
sin A1 4.
A2 4 4 cos2
= 1 1 A.8 sin2 4 2
1 1 1 382 4 8 4 . (Using
A 1sin2 8
)
(iii) 1/ 3s a s b s c
s a3
3s
s a3
4
22
s s. s a3
2s
3 3
2
maxs
3 3 .
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