Find the sum of first 12 even natural numbers.

step 1 Address the formula, input parameters & values.
Input parameters & values:
The number series 2, 4, 6, 8, 10, 12, .  .  .  .  , 24.
The first term a = 2
The common difference d = 2
Total number of terms n = 12

step 2 apply the input parameter values in the AP formula
$$Sum = \frac{n \times (a + Tn)}{2}$$

Where Tn = last term

$$Sum = \frac{12 \times (2 + 24)}{2}$$
$$Sum = \frac{12 \times (26)}{2}$$
$$Sum = \frac{312}{2}$$

$$Sum =156$$
2 + 4 + 6 + 8 + 10 + 12 + .  .  .  .   + 24 = 156

Therefore, 156 is the sum of first 12 even numbers.