cos2b-cos2a
=cos[(a+b)-(a-b)]-cos[(a+b)+(a-b)]
=[cos(a+b)cos(a-b)+sin(a+b)sin(a-b)]-
[cos(a+b)cos(a-b)-sin(a+b)sin(a-b)]
=2sin(a+b)sin(a-b)
=2/3