= radical 2cos2a/(sina+cosa)

= radical number 2 (COSA-SINA) (COSA+SINA)/(SINA+SINA)

= root number 2(cosa-sina)

And cos(π/4-a)= 12/ 13, and 0.

∴sin(π/4-a)= radical sign (1-(12/13) 2) = 5/13.

Namely: sinπ/4cosa-cosπ/4sina = 5/13.

∴cosa-sina=5 root number 2/ 13

∴cos2a/sin(a+π/4)= 10/ 13

The answer is 10/ 13.

Hope to adopt! Please ask questions if you have any questions!