tan(a+b)=(tana+tanb)/(1-tana*tanb),得到tana+tanb=7/3;tana+tanb=(sina/cosa)+(sinb/cosb)=(sinacosb+sinbcosa)/(cosa*cosb)=sin(a+b)/(cosa*cosb),tan(a+b)=7可得sin(a+b)=（+-）（7*√2）/10,所以cosa*cosb=（+-）（3*√2）/10,cos(a-b)=cosacosb+sinasinb=cosacosb(1+tanatanb)=（+-）（3*√2）/10×5/3=（+-）√2/2