Question

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Answer 1

cos^2 a + sin^2 a = cos 2a

1 = cos 2a [cos^2 a + sin^2 a =1]

Since, cos 0 = 1

Therefore, 2a = 0

a = 0

Answer 2

We know,

Cos(a+b)= cos(a)cos(b) - sin(a)sin(b)

Cos(2a)= cos(a+a)= cos(a)cos(a) - sin(a)sin(a)

= cos^2(a) - sin^2(a)

Hence, proved.

Hope you have understood.