Reiny
I do not understand where you got the numbers from in the sigma question on Sunday . Where did you get the number 27 from and where did you get -10. What is the formula you used.
sigma upper limit 28 algebraic expression (8i-13) lower limit 1 index=i
28
∑ 8i-13
i=1
You can see from the formula that the sum increases by 8 with each additional term (8i)
The first term is 8*1-13 = -5
Now recall that the sum of the 1st n terms of an arithmetic sequence is
Sn = n/2 (2a + (n-1)d)
so, with a = -5 and d = 8,
S28 = 28/2 (2(-10) + 8(27))