Question

17.
cos A -sin A
= sec 2A-tan 2A.
cos A + sin A​

Answer 1

If it is sec^2A - tan^2A then it is an trigonometric identity that sec^2A - tan^2A =1 , so you have cos A - sin A = 1 , now square on both the sides , you will get sin^2A + cos^2A - 2sinA.cosA = 1 now sin^2A + cos^2A= 1 (trigonometric identity) therefore , sinA.cosA = 0 , now add 4 times sinA.cosA to sin^2A + cos^2A - 2 cosA.sinA which is equal to 1 , therefore now you will have sin^2A + cos^2A + 2sinA.cosA =1 , now square root on both the side , you will get cosA+sinA=1