已知cos(a+π/4)=3/5,π/2

Question

已知cos(a+π/4)=3/5,π/2

Answer 1

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