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Deepak Bora
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Which term of the A.P. 3, 8, 13, 18, … is 78?

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In this question the last term of A.P. is given, you have to find which term of A.P. this must be. This is question number 4 exercise 5.2,NCERT Maths solutions.

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  1. Solution:

     

    11th term, a11 = 38

    and 16th term, a16 = 73

    We know ,

    an = a + (n−1)d

    a11 = a + (11−1)d

    38 = a + 10d ………………………………. (i)

    By the same way,

    a16 = a +(16−1)d

    73 = a+15d ………………………………………… (ii)

    On subtracting eq. (i) from (ii),

    35 = 5d

    d = 7

    From eq.  (i),

    38 = a+10×(7)

    38 − 70 = a

    a = −32

    a31 = a +(31−1) d

    = − 32 + 30 (7)

    = − 32 + 210

    = 178

    So, 31st term is 178.

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