There is no real shortcut in learning integration formulas. You should take time learning and understanding how the formulas are derived until you are familiar with the formulas and you can easily recall the formulas. Hope this answers the question.
First lets simplify the first term and yield (2a)
The second term:
1. 2a * (-1/2) = -a
2. -a + a = 0
Now we have simplified both terms to: (2a) (0)
Anything multiplied by 0 is 0, so the answer is also 0.
They must conform with Pythagoras theorem
It is set 4 because
(sqrt 2)^2 + (sqrt 7)^2 = 3^2
2 + 7 = 9
