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maths

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A brick wall is built by using 2 bricks less in each layer than in the previous one. The wall has m layers and in total 21m+40 bricks are used. If the top layer has only 4 bricks, determine:A) the value of m
b) the total number of bricks in the wall

  • maths - ,

    i need to know how to do this one please

  • maths - ,

    let a = 4
    we know d = 2
    Sm = m/2[8 + (m-1)(2)]
    21m + 40 = m[3+m]

    take over, (I got m=20)

    Once you have m, sub into 21m+40 for the sum.

  • maths - ,

    thx alot

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