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