I have to find f'(x) and f'(1) for f(x) = cos(x) - 2tan(x)

f'(x) = -sin(x) - 2sec^2(x)

finding values of x I evaluate sin^(-1)(x), in this case sin^(-1)(1), am I right?

f'(1) = -sin(x) - 2sec^2(x) = -sin(x) - 2(1+ cot^2(1)) = -sin(1) - 2(1+ 1/tan^2(1)) = -1.570796327 - 2(1+1/0.616850275) = .....

I don't get where is my mistake, can someone show me?

Thanks