Q. 74.3( 6 Votes )

The Sum of

Answer :

Given: the sum of first three terms is 21


To find: the first three terms of AP


Assume the first three terms are a - d, a, a + d where a is the first term and d is the common difference


So, sum of first three terms is a - d + a + a + d = 21


3a = 21


a = 7


it is also given that product of first and third term exceeds the second by 6


so (a - d)(a + d) - a = 6


a2 - d2 - a = 6


substituting a = 7


72 - d2 - 7 = 6


d2 = 36


d = 6 or d = - 6


Hence the terms of AP are a - d, a, a + d which is 1, 7, 13 or 13, 7, 1

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