Question

Find the sum of all odd numbers between 100 and 150.

Answer 1

Odd numbers between 100 and 150 are 101, 103, ..., 149.

Here $$a = 101, \ d = 2, \ l = 149$$. 

$$n = \frac{149 - 101}{2} + 1 = 25$$. 

Sum $$S_{25} = \frac{25}{2}(101 + 149)$$

$$= 25(125)$$

$$= 3125$$