Volume

Geometry Level 2

If the largest right circular cylinder is cut from a cube with side a a , what is the ratio of the volume of the wasted material to the volume of the cylinder?

π 4 a 3 \frac{\pi}{4}a^3 4 π 1 \frac{4}{\pi} - 1 None of these choices a 3 ( 1 π 4 ) a^3(1 - \frac{\pi}{4})

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2 solutions

Marta Reece
Mar 28, 2017

This is really a two dimensional problem, as the same height multiplies everything. So all we need to deal with is a square and a circle.

We can arbitrarily set the radius of the circle as 1 (since we are only interested in ratios of sizes not the actual values). Area A c i r c l e = π A_{circle}=\pi .

Side of the square will be the diameter, 2, giving us the area A s q u a r e = 4 A_{square}=4 .

So the ratio will be 4 π π = 4 π 1 \frac{4-\pi}{\pi}=\frac{4}{\pi}-1

Solving for the volume of the cube

V c u b e = a 3 V_{cube} = a^3

Solving for the volume of the cylinder

V c y l i n d e r = A b a s e h = π 4 ( a 2 ) ( a ) = π 4 a 3 V_{cylinder} = A_{base}h = \frac{π}{4}(a^2)(a) = \frac{π}{4}a^3

Solving for the volume of the wasted material

V w a s t e d m a t e r i a l = V c u b e V c y l i n d e r = a 3 π 4 a 3 = a 3 ( 1 π 4 ) V_{wasted-material} = V_{cube} - V_{cylinder} = a^3 - \frac{π}{4}a^3 = a^3(1 - \frac{π}{4})

Solving for the ratio

V w a s t e d m a t e r i a l V c y l i n d e r = \frac{V_{wasted-material}}{V_{cylinder}} = a 3 ( 1 π 4 ) π 4 ( a 3 ) = 4 π 1 \frac{a^3(1-\frac{π}{4})}{\frac{π}{4}(a^3)} = \frac{4}{π} - 1

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