Question

there are 15aps whose comman difference are 1,2,3and. 15 respectively then the first term of each being 1 . then sum of their 15th terms is​

Answer 1

The answer is

$\frac{15 \times 16}{2} \\ = 120$

we know that the sum of n number is

$\frac{n \times (n + 1)}{2}$