Question

if last term of an ap is 2n+1 .find sum of 1st 3 terms

Answer 1

if last term of an ap is 2n+1 .find sum of 1st 3 terms

Answer 2

Answer:

Step-by-step explanation:

last term is nothing but the n th term so

an=2n+1

a1=2(1)+3 = 5

a2=2(2)+3 = 7

a=5; d=2

so

$s{n}  =\frac{n}{2}(2a+(n-1)d)\\s_{3} =\frac{3}{2} (10+4)\\=> \frac{3}{2}(14)\\=>7*3 \\=>21$

so now sum of the first 3 terms is 21