Question

If an=3n+2 find the sum of first 20 term's . ​

Answer 1

Answer: The sum of first 20 terms is 670

Step-by-step explanation:

Given,

an=3n+2

To find first term,

a1=(3*1)+2=3+2=5

a2=(3*2)+2=6+2=8

a3=(3*3)+2=9+2=11

This is the series 5,8,11

Common difference d=11-8=3

To find the sum of nth term using the formula,

Sn=n/2[2a+(n-1)d]

S20=20/2*[(2*5)+(20-1)3]

=10*[10+(19)3]

=10*[10+57]

=10*67

=670

Hence, we get the sum of first 20 terms is 670