Question

The sum of all two digit natural numbers is 4905. Prove it

Answer 1

Answer:

two digit natural numbers= 10,11,12,13,14,15,16,17,...,99

a=10

d=1

n=( 99-10)+1 = 90

sum= 90/2(20+89)

= (90×109)/2= 45×109=4905