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Deepak Bora
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If the 5th and 12th terms of an AP are -4 and -18 respectively, find the sum of first 20 terms of the AP.

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Here the two terms of an AP is given you have to find the sum of first n term of the AP
This is the Important question based on Arithmetic progression Chapter of R.S Aggarwal book for ICSE & CBSE Board.
This is the Question Number 18 Of Exercise 11 C of RS Aggarwal Solution

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  1. t5 = – 4

    t12 = – 18

    so,

    a+4d = – 4………1)

    a+11d = – 18…….2)

    on subtracting 1 from 2 we have:

    7d = – 14

    d = – 2

    on putting value of d in eq 1 we have:

    a = 4

    now,

    S20 = 20/2{2a+(20-1)d}

    = 10{8-38}

    = 10 {-30}

    = – 300

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