Q11 of 178 Page 559

The 5th term of an AP is 20 and the sum of its 7th and 11th terms is 64. The common difference of the AP is

Let a be the first term and d be the common difference.


Given: a5 = 20


a7 + a11 = 64


Now, Consider a5 = 20


⇒ a + 4d = 20 ……………(1)


Consider a7 + a11 = 64


⇒ a + 6d + a + 10d = 64


⇒ 2a + 16d = 64 …………(2)


Subtract twice of equation (1) from (2), we get,


8d = 24


⇒ d = 3

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