Sheila has enough paint to cover 16 square feet. She uses some of the paint to cover an x-square-foot canvas. She will then use all of the remaining paint to paint a square with sides that are y feet long. The side length of the square Sheila will paint is a function of the area of the canvas she paints first. Consider this complete graph of the function.

geometry: triangles

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call the larger triangle abc

then the smaller triangle cde is similar to abc (all angles are equal, aaa).  

ab is the side that is 36, and we wish to find de, the side corresponding to ab

area_cde = area_abc - area_cde --> area_abc / area_cde = 2

let x=length of de

ratio of areas = (ratio of sides)^2

+2+=+%2836%2fx%29%5e2+

+2+=+1296%2fx%5e2+

++x%5e2+=+1296%2f2+=+648+

25.45584

15 / 2 = 7.5

100 / 7.5 = 13.