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If a=2 and d=8 and Sn=90 …

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If a=2 and d=8 and Sn=90 . Find n and an.

    • Posted by Salman Yadav 6 years ago

    CBSE > Class 10 > Mathematics
    • 4 answers

    Gaurav Kumar 6 years ago

    Sn=n/2(2a+(n-1)d) 90=n/2(4+8n-8) 180=n(8n-4) 8n²-4n-180 2(4n²-n-45) n=-9/2,n=5

    0Thank You

    Arpit Maurya 6 years ago

    90 = n/2(4+(n-1)8) 180= n(4+8n -8) 180= -4n+8n^2 8n^2-4n -180 =0 Solve it for n

    0Thank You

    Yogita Ingle 6 years ago

    Sn = 90
    Sn = n/2 [ 2a + (n - 1)d]

    90 = n/2 [ 2 x 2 + (n - 1)8]
    180/n = 4 + 8n - 8
    180/n = 8n - 4
    180 = 8n^2 - 4n
    {tex}8n^2 - 4n - 180 = 0{/tex}
    {tex}2n^2 - n - 45 = 0{/tex}
    {tex}2n^2 - 10n + 9n - 45 = 0{/tex}
    2n( n - 5) + 9( n - 5) = 0
    (2n + 9)(n - 5) = 0

    n = -9/2 or n = 5
    But n cannot be negative.
    So, n = 5
    an = a + (n - 1)d
    a5 = 2 + (5-1)8
    a5 = 2 + 4 x 8
    a5 = 2 + 32 = 34

    2Thank You

    Siva Kumar 6 years ago

    N=5,an=34;if n=9/2,an=38

    0Thank You

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