解由a为二象限角,tan(a+π/4)=1/2知a+π/4仍为二象限角,则由sin²（a+π/4）+cos²（a+π/4）=1即tan²（a+π/4）+1=1/cos²（a+π/4）即cos²（a+π/4）=1/(tan²（a+π/4）+1)=1/((1/2)&#...