Question

cos2x=-1???????????????????

Answer 1

cos(2x)=cos²(x)-sin²(x)=2cos²(x)-1=1-sin²(x)

cos(2x)=(1-tg²(x))/(1+tg²(x))=(ctg²(x)-1)/(ctg²(x)+1)=(ctg(x)-tg(x))/(ctg(x)+tg(x))