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Rajan@2021
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How many terms of the A.P; 24,21,18,… must be taken such that their sum is 78

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An important question from ML Aggarwal(Avichal Publication)

Find the number of terms of the A.P; 24,21,18,... must be taken such that their sum is 78.

Sir please give me a detailed solution of this question as it is from Arithmetic Progression (chapter 9)

 

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1 Answer

  1. Sn=2n[2a+(n1)d]

    Given,

    a=24,d=2124=3,Sn=78

    78=2n[2(24)+(n1)(3)]

    156=n[483n+3]

    3n251n+156=0

    n217n+52=0

    (n13)(n4)=0

    n=4,13

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