Q. 45.0( 1 Vote )

Find the sum of 20th and 25th terms of A.P. 2, 5, 8, 11, ...

Answer :

Given: 2, 5, 8, 11, …


Here, a = 2


d = a2 – a1 = 5 – 2 = 3


and n = 20


We have,


tn = a + (n – 1)d


t20 = 2 + (20 – 1)(3)


t20 = 2 + 19 × 3


= 2 + 57


t20 = 59


Now, n = 25


t25 = 2 + (25 – 1)(3)


t25 = 2 + 24 × 3


t25 = 2 + 72


t25 = 74


The sum of 20th and 25th terms of AP = t20 + t25 = 59 + 74 = 133


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