Question

Find the sum of the first 90 terms of the sequence –4, –1, 2, 5, 8

Answer 1

–4, –1, 2, 5, 8
Here first term t1 = -4
common difference d= -1-(-4) =-1+4= 3 and n= 90
sum of nth term of an AP is given as:

\[\huge S_n = \frac{n}{2} [2a+(n-1)d]\]
Substituting the above values
\[\huge S_{90} = \frac{90}{2} [2(-4)+(90-1)3]\]
\[\huge \rightarrow S_{90} = 45 [-8+89 \times 3]\]
\[\huge \rightarrow S_{90} = 45 [-8+267] = 45 \times 259\]
\[\huge \rightarrow S_{90} =11655\]

@AlexaNicole504

Answer 2

here t1 =a