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If cos⁡2A+cos⁡4A=1 \cos^2 A + \cos^4 A = 1cos2A+cos4A=1​, then the value of the expression sin⁡A+sin⁡2A \sin A + \sin^2 AsinA+sin2A​ is:
Question

If cos2A+cos4A=1 \cos^2 A + \cos^4 A = 1​, then the value of the expression sinA+sin2A \sin A + \sin^2 A​ is:

A.

​​​12\frac{1}{2} \\​​

B.

1

C.

2

D.

3

Correct option is B

Given​
cos2A+cos4A=1cos4A=1cos2Acos4A=sin2A sinA=cos2A(....1)\cos^2 A + \cos^4 A = 1 \tag{....1} \\\cos^4A = 1 - \cos^2A \\\cos^4A = \sin^2A \\\implies \sin A = \cos^2 A
​​We know the identity: 
sin2A=1cos2A(......2) \sin^2 A = 1 - \cos^2 A \tag{......2}​​​​
Then: sin2A=(cos2A)2=cos4A \sin^2 A = (\cos^2 A)^2 = \cos^4 A​​
Now, calculate the expression:
sinA+sin2A=cos2A+cos4A\sin A + \sin^2 A = \cos^2 A + \cos^4 A​​
From given cos2A+cos4A=1 \quad \cos^2 A + \cos^4 A = 1​​
=>sinA+sin2A=1\Rightarrow \sin A + \sin^2 A = 1​​
Correct answer is (b) 1. 

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