cosa=-1/3,a是第二象限角,sina=2√2/3,
sin(a+B)=1,cos(a+B)=0
cos(2a+B)=cos[(a+B)+a]
=cos(a+B)cosa-sin(a+B)sina
=-2√2/3
或
sin(a+B)=1,a+B=2kπ+π/2
cos(2a+B)=cos[(a+B)+a]
=cos(2kπ+π/2+a)
=cos(π/2+a)
=-sina
=-2√2/3