因为cos（a+π/4）=3/5>0,a∈ [π/2,3π/2]a+π/4∈ [3π/2,7π/4],所以a∈ [5π/4,3π/2]cos（2a+π/2）=2[cos（a+π/4）]^2-1=-7/25a∈ [5π/4,3π/2]2a+π/2∈ [3π,7π/2]sin（2a+π/2）=-24/25cos（2a+π/4）=c...