Question

Find the sum 7 + `10 1/2` + 14 + .................... + 84.

Answer 1

Here, $$a = 7, \ d = 3.5, \ l = 84$$. 

$$T_n = a + (n - 1)d$$ gives $$7 + (n - 1)(3.5) = 84$$

$$\Rightarrow n = 23$$ 

$$S_{23} = \frac{23}{2}(7 + 84)$$

$$= \frac{23}{2}(91)$$

$$= 1046.5$$