Ex3.若锐角a,b满足tana•tanb=13/7,且sin(a-b)=3分之根号下5 求cos(a+b)=?

Ex3.若锐角a,b满足tana•tanb=13/7,且sin(a-b)=3分之根号下5 求cos(a+b)=?
Ex3.若锐角a,b满足tana•tanb=13/7,且sin(a-b)=3分之根号下5 求cos(a+b)=?

Ex3.若锐角a,b满足tana•tanb=13/7,且sin(a-b)=3分之根号下5 求cos(a+b)=?
锐角a,b
cos(a-b)=2/3
cosa*cosb-sina*sinb=2/3
tana•tanb=13/7
sina*sinb/cosa*cosb=13/7
cos(a+b)=cosa*cosb+sina*sinb
联立解个方程