Question

if the nth term of an AP is (5n-3) find the sum of first 20 term of the AP using the formula​

Answer 1

Given:- nth term of ap=(5n-3)

Let's first take out the terms of the given ap.

So,

(5n-3)

For a1 put the value of n as '1'.

a1=5(1)-3=2

For a2 put the value of n as'2'.

a2= 5(2)-3=7

Now for common difference we have,

d=a2-a1

=7-2=5

Now we know the formula sn=n/2[2a+d]

A/q,

S20=20/2[2×2+19×5]

S20=10(4+25)

S20=990

Thus,the sum is 990.

Answer 2

Explanation:

given

n th term = 5n-3

so, 1 St term = a= 5× 1-3(n=1)

=5-3

=2 ... #

2 nd term = 5× 2-3 ( n= 2)

= 10- 3

= 7...#

20th term °= 5×20 - 3 (n=20)

= 100- 3

= 97

## sum of 20 terms = n (a+l)/2

## n = no. of terms

a= 1st term

l = last term

20(2+97)/2

= 10× 99

= 990 (ans).

THERE CAN BE MISTAKE IN CALCULATION .

PLEASE CORRECT IT YOURSELF