Question

Which summation formula represents the series below?

13 + 9 + 5 + 1

I will give 20 points

Answer 1

Explanation:

The summation formula ( arithmetic sequence when the first terme is : A1 = 13

and the coomo diff is d= -4

Sn = n/2(A1 +An)

An = A1+(n-1)d

An = 13 +(n-1)(-4)

An = -4n +17

Sn = n/2(13+17-4n) = n/2(30-4n)

Sn = -2n² +15n

in this exercice : n =4 S4 = -2(4)² +15(4) = -32+60 = 28

verify : 13 + 9 + 5 + 1 =28 ......right