Question

Find the sum 2 + 7 + 12 + 17 + ...... to 19 terms.

Answer 1

Here, $$a = 2,\ d = 5,\ n = 19$$. 

Using $$S_n = \frac{n}{2}[2a + (n - 1)d]$$: 

$$S_{19} = \frac{19}{2}[2(2) + 18(5)]$$

$$= \frac{19}{2}(4 + 90)$$

$$= 19 \times 47$$

$$= 893$$