Question

Find the sum of first 20 terms of of an ap whose first term is 5 common difference is 4

Answer 1

Answer:

i think a+(n-1) d., =5+(20-1) 4=81

Answer 2

Given :-

First term if an AP = a = 5

Common difference = d = 4

No. of terms = n = 20

Formula to find the sum is n/2 (a + l) where l is the last term or An.

so first of all, we needa find the 20th term.

We know that,

An = a + (n - 1)d

An = 5 + (20 - 1)4

An = 5 + (19)4

An = 5 + 76

➡ An = 81

The 20th term of the AP is 81.

Therefore sum of first 20 terms :-

= n/2 (a + An)

= 20/2 (5 + 81)

= 10 × 86

= 860

Hence, the sum of first 20 terms is 860.