SOLUTION: A rectangular parking lot has dimensions 16 metres by 40 metres. The total area of the parking lot is increased by 512 metres by paving an extra strip of uniform width around the e

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Question 118529: A rectangular parking lot has dimensions 16 metres by 40 metres. The total area of the parking lot is increased by 512 metres by paving an extra strip of uniform width around the entire lot. How wide is the strip ?
I tried (x+16)(x+40)=640+512 and found x=-64 and x=8 so width must be 8 metres since it cannot be negative but I only got partial marks:(

Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
The key in this one is "the area is INCREASED by 512 m2.
That means the area of the uniform strip is 512 m2.
If I call the width of the added strip x,
Then going around the parking lot,
512+=+40x+%2B++x%5E2+%2B+16x+%2B+x%5E2+%2B+40x+%2B+x%5E2+%2B+16x+%2B+x%5E2
512+=+4x%5E2+%2B+80x+%2B+32x
512+=+4x%5E2+%2B+112x
subtract 512 from both sides
4x%5E2+%2B+112x+-+512+=+0
x%5E2+%2B+28x+-+128+=+0
solve by compleying the square
x%5E2+%2B+28x+=+128
x%5E2+%2B+28x+%2B+%2828%2F2%29%5E2+=+128+%2B+%28128%2F2%29%5E2
x%5E2+%2B+28x+%2B+196+=+128+%2B+196
%28x+%2B+14%29%5E2+=+324
x+%2B+14+=+18
x+=+4 m answer
check answer
512+=+4x%5E2+%2B+112x
512+=+4%2A16+%2B+112%2A4
512+=+64+%2B+448
512+=+512
OK