from 4,
+3 +5 +7 +9 +11
What I will do is change it to
3 +1 +3 +5 +7 +9 +11
note that the right side is like a AP, first term = 1, difference =
2
hence, summation of AP is n/2 (2a + (n-1)d)
Ans that you should get is:
3 + n/2 (2 + 2n - 2)
final answer: 3 + n^2
But hor, APGP is not in O level syllabus, and you don't need to do
it this way. I just want to play 
