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The 10th and 15th term of an Ap are -5 and -7½ respectively, what is the sum of the first 20 term

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3 answers

  1. shoe full work

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  2. You must learn the main definitions of sequences, since you will need them

    10th term is -5 ----> a + 9d = -5
    15th term is -7.5 ---> a + 14d = -7.5
    subtract them:
    5d = -2.5
    d = -.5
    then in a + 14d = -7.5
    a - 7 = -7.5
    a = -.5

    now you have a and d, using sum(n) = (n/2)(2a + (n-1)d )
    sum(20) = (20/2)(2(-.5) + 19(-.5) )
    = ....

    let me know what you get

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  3. 40

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