Question

Find the sum of numbers between 1 and 81 which are divisible by 4

Answer 1

4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72,76,80

SUM= 840

Answer 2

First we need to know that the numbers between 1 and 81 which are divisible by 4 form an arithmetic progression with comman difference 4 and first term also 4. Now the last term which is divisible by 4 is 80

a=4

Common difference d = 4

an=80

HERE AN=80=l(L)LAST TERM

an=a+(n-1)d

80=4+(n-1)×4

19=n-1

n=20

therefore, sum=n/2 {a+L}

SUM=20/2 (4+80)

=10 (84)

SUM=840