If the 5th and 12th terms of an A.P. are 30 and 65 respectively, what is the sum of the first 20 terms?
assuming an AP whose first term is a and the common difference is d
To find: the sum of the first 20 terms
Given 5th term is 30
So, a + 4d = 30...(i)
Given 12th term is 65
So, a + 11d = 65…(ii)
Solving (i) and (ii) we get
a = 10 and d = 5
we know that the sum of AP is given by the formula:
substituting the values in the above equation
s = 1150