Question

In an A.P. If the 5th and 12th terms are 30 and 65 respectively, what is the sum of first 20 terms?

Answer 1

hope this solution helps you!!!

Answer 2

Sum of first 20 terms = 1150

Step-by-step explanation:

Given: 5th and 12th terms are 30 and 65

Find: sum of first 20 terms

Solution:

Sum of AP = n/2 (2a + (n-1)d)

  n = 20

a5  = a + 4d  = 30 ---------(1)

a12 = a + 11d = 65 ---------(2)

Subtracting (1) from (2), we get: 7d = 35

Therefore d = +5

Substituting d in (1), we get: a +20 = 30

Therefore a = 10

Sum of first 20 terms = 20/2 ( 2*10 + 19(5))

   = 10 (20 + 95)

   = 10 (115)

  = 1150