Question

y= (senx + cosx)2 - 2 senx . cosx

Answer 1

---------------------------
2.senx.cosx = sen2x
-----------------------------
y = (senx + cosx)² - sen2x
-----------------------------
(senx + cosx)² = 
sen²x + 2.senx.cosx + cos²x  
--------------------------------
sen²x + cos²x = 1
-------------------------------
(senx + cosx)² = 
sen²x + 2.senx.cosx + cos²x = 
2.senx.cosx + 1  = 
sen2x + 1

------------------------------

então:
y = sen2x + 1 - sen2x
y = 1