Trigonometric-Identities Question 389
Question: If $ \sin A+\sin 2A=x $ and $ \cos A+\cos 2A=y, $ then $ (x^{2}+y^{2})(x^{2}+y^{2}-3)= $
Options:
A) $ 2y $
B) $ y $
C) $ 3y $
D) None of these
Show Answer
Answer:
Correct Answer: A
Solution:
Squaring and adding, we get $ x^{2}+y^{2}=1+1+2,\cos ,(2A-A) $
$ \therefore ,\frac{x^{2}+y^{2}-2}{2}=\cos ,A $ ?..(i) Also $ \cos A+2,{{\cos }^{2}}A-1=y $ or $ (\cos A+1),(2,\cos A-1)=y $ Put for cos A from (i) and get the answer.
BETA
