cos(pi/2+A)=5/13,那么cos2A=?

问题描述:

cos(pi/2+A)=5/13,那么cos2A=?

cos(π/2+A)=5/13
cos[π/2-(-A)] = 5/13
sin(-A) = 5/13
sinA = - 5/13
cos2A = 1 - 2sin^2A = 1 - 2*(-5/13)^2 = 119/169