第 11 类 RD Sharma 解决方案 - 第 10 章正弦和余弦公式及其应用 - 练习 10.1 |设置 1
问题1:如果在△ABC中,∠A=45°,∠B=60°,∠C=75°,求其边的比值。
解决方案:
According to the sine rule
Hence, we get
Using the formula,
sin (A+B) = sin A cos B + cos A sin B
sin (45°+30°) = sin(45°) cos(30°) + cos(45°) sin(30°)
sin 75° =
sin 75° =
Multiplying the denominator by 2√2, we get
Hence, we get
a : b : c = 2 : √6 : (√3+1)
问题2:如果在△ABC中,∠C=105°,∠B=45°,a=2,求b。
解决方案:
In the given condition, ∠C=105° and ∠B=45°
And as we know that,
A+B+C = π (Sum of all angles in triangle is supplementary)
A = π-(B+C)
A = 180°-(45°+105°)
A = 30°
Now, according the sine rule
After rationalizing the denominator, we get
问题3:在△ABC中,如果a=18,b=24,c=30,∠C=90°,求sin A,sin B,sin C。
解决方案:
In the given condition, a=18, b=24 and c=30 and ∠C=90°
Now, according the sine rule
Also,
问题4:在△ABC中,证明如下:
解决方案:
According to the sine rule
Considering the LHS of the equation, we have
By using trigonometric formula,
sin A – sin B = 2 sincos
sin A + sin B = 2 sincos
As, LHS = RHS
Hence, proved !!
问题 5:在△ABC中,证明如下:
(ab) = c
解决方案:
According to the sine rule
Considering LHS, we have
By using trigonometric formula,
sin A – sin B = 2 sincos
= λ
A+B+C = π (Sum of all angles in triangle is supplementary)
= 2λ sincoscos
= 2λ sincoscos
= λ sin(2cossin)
By using trigonometric formula,
2 sin a cos a = sin 2a
= λ sin(sin)
= λ sinsin(A+B)
= λ sinsin(π-C) (A+B+C = π)
= λ sinsin(C)
= (λ sin C) sin
= c sin
As, LHS = RHS
Hence Proved!
问题6:在△ABC中,证明如下:
解决方案:
According to the sine rule
Considering LHS, we have
By using trigonometric identities,
sin A – sin B = 2 sincos
sin 2a = 2 sin a cos a
……………………….(1)
Now considering RHS, we have
Cross multiplying we get,
By using trigonometric identities,
sin a cos b + cos a sin b = sin (a+b)
sin a cos b – cos a sin b = sin (a-b)
……………………….(2)
As, LHS = RHS
Hence Proved!
问题 7:在△ABC中,证明如下:
解决方案:
According to the sine rule
Considering LHS, we have
By using trigonometric identities,
sin A + sin B = 2 sincos
sin 2a = 2 sin a cos a
………………………….(1)
Now considering RHS, we have
Cross multiplying we get,
By using trigonometric identities,
cos a cos b + sin a sin b = cos (a-b)
cos a cos b – sin a sin b = cos (a+b)
……………………….(2)
As, LHS = RHS
Hence, Proved!
问题8:在△ABC中,证明如下:
解决方案:
According to the sine rule
Considering LHS, we have
By using trigonometric identities,
sin A + sin B = 2 sincos
sin 2a = 2 sin a cos a
As, LHS = RHS
Hence, Proved!
问题 9:在△ABC中,证明如下:
解决方案:
According to the sine rule
Considering RHS, we have
By using trigonometric identities,
sin A – sin B = 2 sincos
By using trigonometric identities,
2 sin a cos a = sin 2a
As, LHS = RHS
Hence, Proved!
问题 10:在△ABC 中,证明如下:
解决方案:
According to the sine rule
Considering LHS, we have
By using trigonometric identities,
sin2 a – sin2 b = sin(a+b) sin (a-b)
As, LHS = RHS
Hence, Proved!
问题 11:在△ABC中,证明如下:
b sin B – c sin C = a sin (BC)
解决方案:
According to the sine rule
And, cosine rule,
Considering RHS, we have
RHS = a sin (B-C)
By using trigonometric identities,
sin(a-b) = sin a cos b – cos a sin b
= a (sin B cos C – cos B sin C)
= a ((bλ)–(cλ))
= λ
= 2λ
= λb2-λc2
= b(λb) – c(λc)
= b(sin B) – c(cos C)
As, LHS = RHS
Hence, Proved!
问题 12:在△ABC中,证明如下:
a 2罪 (BC) = (b 2 -c 2 ) 罪 A
解决方案:
According to the sine rule
Considering RHS, we have
RHS = (b2-c2) sin A
= λ2 sin A(sin2 B – sin2 C)
By using trigonometric identities,
sin2 a – sin2 b = sin(a+b) sin (a-b)
= λ2 sin A(sin(B+C) sin (B-C))
= λ2 sin A(sin(\pi-A) sin (B-C))
= λ2 sin A(sin(A) sin (B-C))
= λ2 sin2 A sin (B-C)
= (λ sin A)2 sin (B-C)
= a2 sin (B-C)
As, LHS = RHS
Hence, Proved!
问题 13:在△ABC中,证明如下:
解决方案:
Considering LHS, we have
LHS =
Rationalizing the denominator, we get
According to the sine rule
As, LHS = RHS
Hence, Proved!
问题 14:在△ABC中,证明如下:
a (sin B – sin C) + b(sin C – sin A) + c(sin A – sin B) = 0
解决方案:
According to the sine rule
Considering LHS, we have
LHS = a (sin B – sin C) + b(sin C – sin A) + c(sin A – sin B)
= λ sin A (sin B – sin C) + λ sin B (sin C – sin A) + λ sin C (sin A – sin B)
= λ sin A sin B – λ sin A sin C + λ sin B sin C – λ sin B sin A) + λ sin C sin A – λ sin C sin B
= 0
As, LHS = RHS
Hence, Proved!
问题 15:在△ABC中,证明如下:
解决方案:
According to the sine rule
Considering LHS, we have
LHS =
=
= λ2 sin A sin (B-C) + λ2 sin B sin (C-A) + λ2 sin C sin (A-B)
By using trigonometric identities,
sin(a-b) = sin a cos b – cos a sin b
= λ2 (sin A [sin B cos C – cos B sin C] + sin B [sin C cos A – cos C sin A] + sin C [sin A cos B – cos A sin B])
= λ2 (sin A sin B cos C – sin A cos B sin C + sin B sin C cos A – sin B cos C sin A + sin C sin A cos B – sin C cos A sin B)
= λ2 (0)
= 0
As, LHS = RHS
Hence, Proved!