Question

find the sum of all two digit numbers​

Answer 1

Answer::

Sn = n/2 [a+l]

first 2 digit number (a) = 10, last number(l) = 99, no.of digits from 10 to 99 = 90

Sn = 90/2 [10+99]

Sn = 45(109)

Sn = 4905