Question

Find th sum of 7+12+17+22+..............+102

Answer 1

let a=7 and common difference d=(12-7)=5
and last term = L=102
THEN ,
L=A+(N-1)D
=>102=7+(N-1)5
=>102-7=(N-1)5
=>(N-1)=95/5
=>N=19+1 =20
then, we get N=20
NOW THE SUM IS,
Using the sum formula ,
Sₙ=N/2[A+L]
because here we know the last term, then we will aply this formula
=>S₂₀=20/2[7+102]
=>20/2(109)
=>10*109
hence,S₂₀=1090

I hope you understand very well dear, : - )