math

if the 7th term of an a.p is twice the third term and the sum of the first four terms is-16 and the last and the last term is 72, find the number of the terms

  1. 👍 1
  2. 👎 0
  3. 👁 55
asked by akorede
  1. show me the answer

    1. 👍 0
    2. 👎 1
    posted by akorede
  2. Let an be the nth term of arithmetic progression a. Can you then start to write out your ap using symbols?

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    posted by Leo
  3. not so.
    I'll show you how to find the answer.
    Using the formula for the nth term of an AP, you know that
    a+6d = 2(a+2d)
    4/2 (2a+3d) = 16
    Now you can find a and d, so you just need to know how many terms there are.
    If there are k terms, then
    a+(k-1)d = 72
    so using a and d found in part 1, solve for k.

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    posted by oobleck

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