Question

Find the sum of the first 9 terms of the sequence.

2, -7, -16, -25, ...

Answer 1

do I have to find the last term first?

Answer 2

l=a + (n-1)d , where l= last term, a= first term, n= # of terms, d= difference

Answer 3

@jdoe0001

Answer 4

dont you just find the pattern, find 1st 9 num, then add?

Answer 5

help, no the

Answer 6

sigma

Answer 7

\[\Sigma \]

Answer 8

This is obviously an arithmetic series.
To find the common difference, subtract the first term from the second term...
-7 - 2 = - 9....so the common difference is -9.

Now before we find the sum, we need to find the 9th term
an = a1 + (n-1) * d
n = term we want to find = 9
a1 = first term = 2
d = common difference = -9
now we sub
a9 = 2 + (9-1) * -9
a9 = 2 + 8 * -9
a9 = 2 - 72
a9 = - 70

now that we know the 9th term we will then find the sum
sn = n(a1 + an) / 2
n = terms we need to add = 9
a1 = first term = 2
a9 = -70
now we sub
s9 = 9 (2 - 70) / 2
s9 = 9 (-68) / 2
s9 = 9 * - 34
s9 = - 306

so the sum of the first 9 terms is -306

Answer 9

yay thanks<3