已知：0
问题描述：
已知：01.求sin2 β的值
2.求cos (a +派/4）的值
答
cos（π/2+α）= -sinαcos ( β+π/4）=1/3,sin (a+ β)=4/5cos ( β+π/4）=1/3cos (2β+π/2)=2cos²(β+π/4)-1cos (2β+π/2)=2*(1/3)²-1cos (2β+π/2)=2/9-1cos (2β+π/2)=-7/9-sinα2β=-7/9sinα...