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Question 13 : Let Sn be sum of the first n terms of an A.P. {an }. If S5 = S9 , what is the ratio of a3 : a5
Let Sn be the sum of the first n terms of an A.P.
It is given that S5 = S9
a1 + a2 + a3 +a4 + a5 = a1 + a2 + a3 +a4 + a5 + a6 + a7 + a8 + a9
a6 + a7 + a8 + a9 = 0
Let us suppose ‘a’ be the middle term of the a7 and a8
And let us supposea8 - a7 = 2d than a - a7 = d and a8 - a = d
a7 = a - d, a8 = a + d
a6 = a - 3d and a9 = a + 3d
We know that a6 + a7 + a8 + a9 = 0. If we substitute the above values, we will get
a = 0
So that, the values of a1 , a2 , a3 , a4 , a5 , a6 , a7 , a8 , a9 becomes -13d, -11d, -9d, -7d, -5d, -3d, -d, d, 3d
Hence a3 : a5 = 9 : 5
The question is " Let Sn be sum of the first n terms of an A.P. {an }. If S5 = S9 , what is the ratio of a3 : a5 "
Choice A is the correct answer.
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