sin（a+π/2）=cosa=1/3,cos2a=2cos^2a-1=2/9-1=-7/9
sin（a+π/2）=1/3 ∴cos（a）＝1/3∴ cos2a=2cos²a-1=2(1/3)²＝2/9-1=-7/9
sin(a+π/2)=1/3cosa=1/3cos2a=2（cosa）^2-1=2/9-1=-7/9
