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January 28, 2015

January 28, 2015

Posted by **Riana** on Sunday, January 27, 2013 at 1:42am.

- math help! -
**Stephen**, Sunday, January 27, 2013 at 7:37amSolve using aritmetic progression and quadratic equation, how?

Sum of AP = n/2(2a+(n-1)d)

Where n is the number of terms, ie in your case the number of rows of seats, 'a' is the number in the first row (a=262) and 'd' is the difference in the next row (d=18). Sum of AP is the number of seats in the arena (Sum=15690). Put this together and you get:

15690=n/2[2*262+(n-1)18]

18n^2+506n-31380=0

equation to: ax^2+bx+c,; a=18, b=506, c=-31380

Now comes the quadratic, solution to a quadratic =

[-b(+/-)Sqrt(b^2-4ac)]/2a

substitute the values of a,b anc from above into the quadratic and solve.

n=30.

Please always check your answer, in this case use sum of AP.

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