Question

Find the sum of all even numbers between 10 and 100.

Answer 1

Even numbers between 10 and 100 are 12, 14, ..., 98. 

Here $$a = 12, \ d = 2, \ l = 98$$.

$$n = \frac{98 - 12}{2} + 1 = 44$$. 

Sum $$S_{44} = \frac{44}{2}(12 + 98)$$

$$= 22(110)$$

$$= 2420$$