Question

if the 3rd and the 6th terms of an A.P. are 7 and 13 respectively,find the sum of the first 20 terms?

Answer 1

solve with
$an = a + (n - 1) \times d$

Answer 2

a3=7
a6=13

S20=?

a3=7

=>a+2d=7..........¡


a6=13

=>a+5d=13..........¡¡


¡¡-¡ => 3d=6

=>d=2

Now putting d=2 in equation ¡ we get

a+2d=7

=>a+2×2=7

=>a=7-4

=>a=3

Now S20=n/2{2a+(n-1)d}

=>S20=10{2×3+19×2}

=>S20=10×44

=>S20=440