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Arithmetic Progression Difficulty

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Why bother with a post grad? Are they even worth it? Have your say! 26-10-2016
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    Someone invests £1000 in an account offering an interest rate of 5% each year. How many years will it take to get £1500.

    I used the equation Sn=n/2[2a+(n-1)d] and I have made d=1.05, a=1000 and Sn=1500.

    I got a quadratic equation which came out at about 1.5 and -2000 which can't be right.

    So have I made a glaring error or used the wrong equation?

    Any help/advice would be great.


    (Original post by Pokeyguy20)
    So have I made a glaring error or used the wrong equation?
    Well, you've used the sum of an A.P., whereas I think you want the n'th term in a G.P.
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