Question

Find the sum of 5+8+11to ten terms using the formul

Answer 1

Answer:

Given numbers form an AP, with -

First term = a = 5

common diff. = d = 11 - 8 = 8 - 5 = 3

We know, in AP,

S = $\dfrac{n}{2} \{ 2a + (n-1)d\}$,

d is the common difference

S = $\dfrac{n}{2} \{ a + l\}$

Here,

we are not given with the last term (l), so we will apply the 1st formula-

a = 5

d = 3

n = 10

Hence,

Sum of terms -

= > $\dfrac{10}{2} \{ 2(5) + (10-1)3 \}$

= > 5( 10 + (9)3 )

= > 5(10 + 27)

= > 5(37)

= > 185

Sum of first 18 terms is 185