cos(a+b)cos(a-b)=1/5,则(cosa)^2-(sinb)^2=?

1/5.
cos(a+b)cos(a-b)=(cosacosb-sinasinb)(cosacosb+sinasinb)
=(cosacosb)^2-(sinasinb)^2
=cosa^2[1-(sinab)^2]-sinb^2[1-(cosa)^2]
=(cosa)^2-(sinb)^2
=1/5