Question

Find the sum of given terms: -

81 + 82 + 83 + …. + 89 + 90

Answer 1

Given series 81 + 82 + 83 + …. + 89 + 90

∴ T_n = I = a + (n − 1)d

∴ 90 = 81 + (n − 1). 1

(here, a = 81, I = 90 and d = 1)

⇒ n = 9

Now,

S_n = \(\frac{n}{2}\)[2a + (n − 1)d]

= \(\frac{10}{2}\)[2 × 81 + (10 − 1) × 1]

= 5[162 + 9]

= 5 × 171 = 855