Answer:
Step-by-step 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
