Q7 of 105 Page 146

Show that a1, a2,…..an form an AP where is defined as below :

an = 9-5n


Also find the sum of the first 15 terms in each case.

Given an = 9-5n

So, a1 = 9 – 5(1) = 4


a2 = 9 -5(2) = -1


a3 = 9 – 5(3) = -6


as of now a1, a2, a3 have the same common difference


so given one is A.P


The sum of first 15 terms is given by (sn) = [2a + (n-1) d]


= [ 2×4 +(15-1) (-5)]


= -465


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