12= 96 the sum of the A. P. 3, 7,11,-.is 210, the number of terms C) 15 22 62 12 a) 10​

Question

12= 96 the sum of the A. P. 3, 7,11,…..is 210, the
number of terms
C) 15
22
62 12
a) 10​

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Eloise 1 month 2021-10-22T00:43:54+00:00 2 Answers 0 views 0

Answers ( )

    0
    2021-10-22T00:45:15+00:00

    Answer:

    The answer is a option A

    0
    2021-10-22T00:45:18+00:00

    Answer:

    Option A) 10 is the correct answer

    The answer to this question briefly is given below:

    Given things:

    a=3                                    Sn=210

    d= 7-3 =4                             n=?

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

    210= n/2[ 2 x 3+ (n-1)x 4]

    210= n/2[ 6+4n-4]

    210= n/2[2+4n]

    210 x 2= 2n+ 4nx^{2}

    420= 2n+4nx^{2}

    4nx^{2}+2n-420=0

    By using quadratic formula:

    a=4    b=2    c= -420

    d= bx^{2}– 4ac

    d= 2x^{2}– 4 x 4 x-420

    d= 4+6720

    d= 6724\sqrt{x}                   d=82

    n= -b+-\sqrt{x} bx^{2} -4ac ÷ 2a

    n= -2+-82÷ 2 x 4

    n= -2+-82÷8

    n= -2+82÷ 8                          n=-2-82÷8

    n= 80÷8                                 n= -84÷8

    n=10                                       n= 10.5

    According to the no of terms, a decimal number cannot be included in AP

    So that n=10

    I hope this answer will be helpful to you

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