13. a(62+c)cos A +b(co + a2)cos B+c(a2 +62) cos C=3 abc.

bosin C + csin B 16. vbc sin B sin C=

b+c 17. a+b+c= (b + c)cos A +(c + a)cos B +(a + b)cos C. 18. 6+c-a= (b + c)cos A -(-a)cos B +(a - b)cos C.

COS 19. a cos (A+B+C) -b cos (B +A)-COS (A+C)=0.

a+62 +ca 20.

+
b

2 abc

COS A

cos B

cos C

+

a

24. tan

B
2

.

2

(0+0–a)= tan (c+a – b).

-b 25. d*= (a + b)sin+(a — b)? cosa 26. c(cos A + cos B)=2(a + b) sin

in? 27. escos A – cos B)=2(6 – a)cosao

A 28. tan B = tan C=(a? + 62 — c?) = (a’ — + c).

2C
2

29. a2 + b3 + c^= 2(ab cos C + bc cos A + ca cos B).

B • cosa

= ($- a) + b($-b). 2

2

30. cos24

32. If p is the length of the perpendicular from A on

P BC,

sin A

ар.
bc

=

1

33. If A=3 B, then sin B

3b – a

b

then B=C.

basin B + cʻsin C 34. If ✓bc sin B sin C

b+c B С A 35. a cos COS cosec-=S.

2

=

36. If cos A = }, and cos B=1}, then cos C

- 15. 37. If sin’B + sin’C = sin’A, then A = 90°. 38. If D is the middle point of BC, prove that

4 AD' = 212 +2c - a'.

39. If a= 26, and A = 3B, prove that C = 60°.

40. If D, E, F, are the middle points of the sides, BC, CA, AB, prove

4(ADʻ + BE’ + CF”)=3(a’ + b + c). 41. If a, b, c, the sides of a triangle, are in arithmetic progression, prove

A С 1
2 2 3

44. If a = b? — bc+c, prove that A = 60°. ’=

A = 45. If the sides of a triangle are a, b, and va’ + ab + b?, prove that its greatest angle is 120°.

46. Prove that the vertical angle of any triangle is divided by the median which bisects the base, into segments whose sines are inversely proportional to the adjacent sides. 47. If AD be the median that bisects BC, prove (1)

(52 — c) tan ADB = 2 bc sin A, and (2) cot BAD + cot DAC=4cot A + cot B + cot C.

=

48. Find the area of the triangle ABC when a= : 625, b b= 505, c= 904 yards. :

Ans. 151872 sq. yards. 49. Find the radii of the inscribed and each of the escribed circles of the triangle ABC when a=13, b= 14,

Ans. 4; 10.5; 12; 14.

=

c=15.

50. Prove the area S= } a' sin B sin C cosec A. 51.

66

A B
COS

COS COS
2 2

9)

2 abc 52.

a+b+c 53. Prove that the lengths of the sides of the pedal triangle, that is, the triangle formed by joining the feet of the perpendiculars, are a cos A, 6 cos B, c cos C, respectively.

54. Prove that the angles of the pedal triangle are, respectively, 1 – 2 A, – 2 B, 1 – 2 C.

A B 55. Prove rir2r3 = gol cot: cot?

2 2

Cot C

COS

vic

A

B С 56. Prove ri cos

= A cOS 2

2 2 57. Prove that the area of the incircle : area of the tri.

А
B

С angle :: 7 : cot cot cot

:

Prove the following statements :
58. If a, b, c, are in A.P., then ac=6rR.

59. If the altitude of an isosceles triangle is equal to the base, R is five-eighths of the base.

=

60. bc=4R2(cos A + cos B cos C).
61. If C is a right angle, 2r +2R= a + b.
62. r283 + 73?"1 + 1,72= s?.

1 1 1 1
63. + +
bc ca ab 2rR

С 64. ritr2=ccot

a

с

+
+

66. If P1, P2, P3 be the distances to the sides from the circumcentre, then

b

abc +

Pi P2 P3 4 P1P2P3 67. The radius R of the circumcircle

1 3 abc
2 Vsin A sin B sin c

70. abcr = 4R($ - a)(: -b) (s —c).

71. The distances between the centres of the inscribed and escribed circles of the triangle ABC are 4R sin B

C 4R sin 4R sin 2'

2

72. If A is a right angle, ra+r3 = a.

rri.

73. In an equilateral triangle 3 R=6r=271

74. If r, ru, ru, rz denote the radii of the inscribed and escribed circles of a triangle,

A tan?

2 7273 75. The sides of a triangle are in arithmetic progression, and its area is to that of an equilateral triangle of the same perimeter as 3 is to 5. Find the ratio of the sides and the value of the largest angle.

Ans. As 7, 5, 3; 120°. 76. If an equilateral triangle be described with its angular points on the sides of a given right isosceles triangle, and one side parallel to the hypotenuse, its area will be 2 a' sin15° sin 60°, where a is a side of the given triangle.

77. If h be the difference between the sides containing the right angle of a right triangle, and S its area, the diameter of the circumscribing circle = h + 4S.

78. Three circles touch one another externally : prove that the square of the area of the triangle formed by joining their centres is equal to the product of the sum and product of their radii.

79. On the sides of any triangle equilateral triangles are described externally, and their centres are joined: prove that the triangle thus formed is equilateral.

80. If 01, 02, 03 are the centres of the escribed circles of a triangle, then the area of the triangle 0,0,0,= area of

=

b triangle ABC| 1+

+

+ ata

a+b-C 81. If the centres of the three escribed circles of a tri. angle are joined, then the area of the triangle thus formed

abc is where r is the radius of the inscribed circle of the

270 original triangle.

)

a

b + c

a

a