Question

Find the sum of first 20 terms of arithmetic series 2+7+12..... using suitable formula

Answer 1

Given Terms of the AP:
2, 7 , 12.....


Here,
a = 2
and
d=7-2=5

Therefore the 20th term must be,

2 +19(5)
= 2+ 95
=97.


Now,
The sum of first 20 numbers of the AP here will be:

$\frac{20}{2} (2 + 97) \\ = 10 \times 99 \\ = 990$



So your required answer will be 990.

Answer 2

a=2 d=a2-a1. n=20.

=7-2

=5

• Sn=n/2[2a+(n-1)d]

• S20=20/2[2(2)+(20-1)(5)

• =10(4+95)

• =10(99)

• 990

990 is your answer mate.