sina+cosa=1/5 且π/2≤a≤3π/4 则cos2a的值是RT

问题描述:

sina+cosa=1/5 且π/2≤a≤3π/4 则cos2a的值是
RT

sina+cosa=1/5 (π/2≤a≤3π/4)
=>sina=4/5 cosa=-3/5
cos2a=(cosa)^2-(sina)^2=(-3/5)^2-(4/5)^2=9/25-16/25=-7/25