（sinA+cosA)^2=sin2A+1=7/4sin2A=3/4cos[1/2(A+B)]=cos[(A-B/2)-(A/2-B)]=cos(A-B/2)cos((A/2-B)+sin(A-B/2)Sin(A/2-B)=-1/9 *根号5/3 + 根号80/9 *2/3= 7根号5/27 cos（A+B）=2COS^2[1/2(A+B)]-1=-239/729...