Question

In an A.P 5, 15, 25,... find the sum of first 20 terms.​

Answer 1

Question :

In an A.P. 5 , 15 , 25 , ... Find the sum of first 20 terms.

Formula :

$\tt {S}_{n} = \frac{n}{2}(2a + (n-1)d)$

Where,

n. : number of terms

a : first term

d : common difference

Solution :

Given A.P. : 5 , 15 , 25 , . . .

First term : a = 5

Common difference : d = 15 - 5 = 10

No. of terms : n = 20

$\tt {S}_{20} = \frac{20}{2}(2×5 + (20-1)10)$

$\tt \implies {S}_{20}$ = 10(10+19×10)

$\tt \implies {S}_{20}$ = 10 (10 + 190)

$\tt \implies {S}_{20}$ = 10 × 200

$\tt \implies {S}_{20}$ = 2000

Sum of 20 terms is 2000.