Question

Find the sum of the sequence
39+40+41+42+....+138

Answer 1

Firstly, tell me how many terms does this sequence have?

Answer 2

First term a = 39 and last term l = 138

Use:
\[t_n = a + (n-1)d\]

find n from it and then put it into:

\[\huge Sum = \frac{n}{2}(a + l)\]

Answer 3

There, \(t_n = 138\)

Answer 4

138 = 39 + (n-1)(1)

138 = 38 + n

n = 100

Just plug in the formula above:

\[Sum = \frac{100}{2}(39 + 138) = 50 \times 177 = 8850\]