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16 March, 05:53

Find the sum of 1st 20 terms of an AP in which third term is 7and 7th term is 2more than thrice of its 3rd term

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  1. 16 March, 06:14
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    S=740

    Step-by-step explanation:

    a3=7

    a7=3*a3+2=3*7+2=23

    a7=a3+4r

    23=7+4r

    -7 - 7

    16=4r

    r=16/4

    r=4

    a20=a7+13r

    a20=23+13*4

    a20=23+52

    a20=75

    a1=a3-2r

    a1=7-2*4

    a1=7-8

    a1=-1

    S = (a1+a20) * 20/2 = (-1+75) * 10=74*10=740
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