(sin a + cos a)^2 - 2sin a*cos a

Expanding the left side using the formula (a+b)^2 = a^2 + 2ab + b^2:

= sin^2 a + 2sin a cos a + cos^2 a - 2sin a cos a

Now simplify:

= sin^2 a + cos^2 a

Since sin^2 a + cos^2 a = 1 (from the Pythagorean trigonometric identity), the final result is:

= 1