Question

Find the sum of all natural numbers from 51 to 100​

Answer 1

It's an A.P.

$a_1=51\\a_n=100\\d=1\\S_n=\dfrac{a_1+a_n}{2}\cdot n\\n=?\\\\a_n=a_1+(n-1)d\\100=51+(n-1)\cdot 1\\n-1=49\\n=50\\\\S_{50}=\dfrac{51+100}{2}\cdot 50=3775$