how much does this equals? 3sin^(2)x - 7cos^(2)x , when cos2x=-0,1 ?
2sin^2(x) = 1 - cos(2x)
2cos^2(x) = 1 + cos(2x)
plug those into your expression and evaluate when cos(2x) = 0 or 1
2cos^2(x) = 1 + cos(2x)
plug those into your expression and evaluate when cos(2x) = 0 or 1