How much is two plus three plus four plus five plus six?

How much is two plus three plus four plus five plus six?

2+3+4+5+6
=（2+3+5）+（4+6）
=10+10
=20
The answer is not easy. Please understand,

How much is one plus one plus two plus three plus four plus five plus six

1+1+2+...+6
From the second number to the last number is an arithmetic sequence
Summation of arithmetic sequence: (first term + last term) * number of terms / 2 = (1 + 6) * 6 / 2 = 21
Plus one, so it's 22

In the equation x = a times the square of X + BX + C, when x = 1, y = - 2, when x = - 1, y = 20; when x = 3 / 2 and x = 1 / 3, the value of Y is equal!
Finding the value of a, B, C

Generation, a + B + C = - 2, A-B + C = 20 9 / 4A + 3 / 2B = 1 / 9A + 1 / 3b, a = 6, B = - 11, C = 3