Cos photography Dali

0 & ltπ/4-a & lt； π/4

So sin (π/4-a) >: 0.

Sin? (π/4-a)+cos？ (π/4-a)= 1

So sin(π/4-a)=5/ 13.

Then cos2a

=sin(π/2-2a)

=sin[2(π/4-a)]

= 2 sin(π/4 a)cos(π/4 a)

= 120/ 169

sin(π/4+a)

=cos[π/2-(π/4+a)]

=cos(π/4-a)

= 12/ 13

So the original formula = 10/ 13.