**Solution:**

cos(π/4 - x) cos(π/4 - y) - sin(π/4 - x)sin(π/4 - y)

**Using formula: cosA.cosB-sinA.sinB=cos(A+B)**

cos(π/4 - x) cos(π/4 - y) - sin(π/4 - x)sin(π/4 - y)

=cos{(π/4 - x)+(π/4 - y)}

=cos{π/4 - x+π/4 - y}

=cos{π/2 - x - y}

=cos{π/2 -( x + y)}

=sin( x + y)