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Given that T7=23,Tn=43 and T2n=91.for an A.P.find a,d and n...help please

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  1. T7=a+6d=23
    Tn=a+[n-1]d=43
    T2n=a+[2n-1]=91
    subtract equ 2 from equ 3 give
    nd=48---equ 4
    from equ2 nd=43+d-a
    :- 48=43+d-a
    5=d-a ---equ6 recall from equ1 a=23-6d put the value of a into equ 6
    d=4 from equ4 nd=48 since d=4, n=12
    put the value of d into equ1 :- a=-1
    (n=12,d=4,a=-1]

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    posted by joshua
  2. Thank you

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    posted by favour

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