Question

If the sum of first in term cef An AP is a 4n-n2
what is the first term, find the term and
Sum of 15 th term​

Answer 1

Answer:

-165

Step-by-step explanation:

Sn=4n-n²(given)

S1=4(1)-(1)²

a1=4-1

a1=3

first term(a1) is 3.

S2=4(2)-(2)²

a1+a2= 8-4

3+a2=4

a2=1

second term (a2) is 1.

d=a2-a1

=1-3

=-2

common difference is -2.

we know that,

Sn = n/2[2a+(n-1)d]

S15= 15/2[2(3)+(15-1)-2]

= 15/2[6+14(-2)]

=15/2×(6-28)

=15/2×-22

=15×-11

=-165