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Arithmetic Progression (Question)

fixxxer said – Thu, 01 Oct 2009 13:49:31 -0000 ( Link )

The sum of n terms of two series in A.P. are in the ratio of (7n-17):(4n+16). find the ratio of their 21st terms.

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  1. abheyhanda said – Mon, 05 Oct 2009 16:14:40 -0000 ( Link )

    replace n with 41

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  2. deepa mittal said – Thu, 29 Oct 2009 06:15:15 -0000 ( Link )

    The solution can be found out by applying basics of AM.

    Sum of n terms of AP = n *(First Term + last term) / 2 AM of AP = (First Term + last term) / 2

    Hence, Sum of n terms of AP = n * AM

    Now lets come back to the question.

    AM of 41 terms will be the 21st term of the AP.

    Hence, Sum of 41 terms of AP = 41 * (21st term of AP)

    Hence, the ratio of 21st terms of 2 APs will be equal to the ratio of the sum of 41 terms of 2 APs.

    So, put n = 41 and get the answer which would be 3 : 2

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