Homework Help: maths

Posted by Anonymous on Sunday, February 21, 2010 at 8:00am.

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 - Anonymous, Sunday, February 21, 2010 at 9:35am
i need to know how to do this one please
maths - Reiny, Sunday, February 21, 2010 at 9:51am
let a = 4
we know d = 2
S_m = 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 - Anonymous, Sunday, February 21, 2010 at 10:13am
thx alot

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