найдите 4cos2a, если sina=-0,5

Question

найдите 4cos2a, если sina=-0,5

Answer 1

cos2a=cos²a-sin²a

4(cos²a-sin²a)

cos²a=1-sin²a

4(1-sin²a-sin²a)

4(1-2sin²a)

4-8sin²a

4-8* 0.25=2

Answer 2

cos2a=1-2sin^2a = 1-2*0.25=0.5

4cos2a=0.5*4=2