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Lawnmower

Age 14 to 16
Challenge Level Yellow star
  • Problem
  • Student Solutions

Tony Cardell, has sent in this solution, which gives the correct answer. We're still not quite convinced: how does he know that the strips are parallel to the longer side? If anyone can explain this, we'll add their explanation here.

We must find the maximum distance between a pair of opposite sides (it doesn't matter which since this is a kite). If we extend the two sides (coloured blue), the intersection is on the same side of the kite as the equilateral triangle, as shown. So the maximum distance between the two sides is the red line.

Now look at the isosceles triangle part of the kite. One formula for the area of a triangle with sides a,b,c says that if s is the semi-perimeter, (a+b+c)/2, then the area is the square root of s(s-a)(s-b)(s-c). The isosceles triangle has sides 169, 169, 130, so s=234 and the area is 10140 square feet. But we also know that the area is 1/2 x base x height, so the height we want is 10140 x 2 / 169=120. So 120 foot-wide strips will be needed.


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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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