Question

The sum of the first n terms of the arithmetic sequence is 66. What if α₁ = 9 and d = 5, what is n?

Please show complete solution.

Answer 1

Answer:

Sn= 66

A1=9

d=5

an= A1+(n-1)d

an=9+(n-1)5

an=4+5n

Sn=n/2(a+an)

66=n/2(9+4+5n)

132=n(13+5n)

5n²+13n-132=0

5n²+33n-20n-132=0

5n(n-4)+33(n-4)=0

n=4