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Geometry Difficulty 3.0 Junior Find the answer

Rectangle WXYZW X Y Z has WX=4,WZ=3W X=4, W Z=3, and ZV=3Z V=3. The rectangle is curled without overlapping into a cylinder so that sides WZW Z and XYX Y touch each other. In other words, WW touches XX and ZZ touches YY. The shortest distance from WW to VV through the inside of the cylinder can be written in the form a+bπ2cπ2\sqrt{\frac{a+b \pi^{2}}{c \pi^{2}}} where a,ba, b and cc are positive integers. What is the smallest possible value of a+b+ca+b+c?

A number or a short expression. Spacing and $ signs are ignored.

Solution

When the cylinder is created, WW and XX touch and ZZ and YY touch. This means that WYW Y is vertical and so is perpendicular to the plane of the circular base of the cylinder. This means that VYW\triangle V Y W is right-angled at YY. By the Pythagorean Theorem, WV2=WY2+VY2W V^{2}=W Y^{2}+V Y^{2}. Note that WYW Y equals the height of the rectangle, which is 3 (the length of WZW Z) and that VYV Y is now measured through the cylinder, not along the line segment ZYZ Y. Let OO be the centre of the circular base of the cylinder. In the original rectangle, ZY=WX=4Z Y=W X=4 and ZV=3Z V=3, which means that VY=1=14ZYV Y=1=\frac{1}{4} Z Y. This means that VV is one-quarter of the way around the circumference of the circular base from YY back to ZZ. As a result, YOV=90\angle Y O V=90^{\circ}, since 9090^{\circ} is one-quarter of a complete circular angle. Thus, YOV\triangle Y O V is right-angled at OO. By the Pythagorean Theorem, VY2=VO2+OY2V Y^{2}=V O^{2}+O Y^{2}. Since YOY O and OVO V are radii of the circular base, then VO=OYV O=O Y and so YV2=2VO2Y V^{2}=2 V O^{2}. Since the circumference of the circular base is 4 (the original length of ZYZ Y), then if the radius of the base is rr, we have 2πr=42 \pi r=4 and so r=42π=2πr=\frac{4}{2 \pi}=\frac{2}{\pi}. Since VO=rV O=r, then YV2=2VO2=2(2π)2=8π2Y V^{2}=2 V O^{2}=2\left(\frac{2}{\pi}\right)^{2}=\frac{8}{\pi^{2}}. This means that WV2=WY2+YV2=9+8π2=9π2+8π2=8+9π2π2W V^{2}=W Y^{2}+Y V^{2}=9+\frac{8}{\pi^{2}}=\frac{9 \pi^{2}+8}{\pi^{2}}=\frac{8+9 \pi^{2}}{\pi^{2}} and so WV=8+9π21π2W V=\sqrt{\frac{8+9 \pi^{2}}{1 \cdot \pi^{2}}}. Since the coefficient of π2\pi^{2} in the denominator is 1, it is not possible to 'reduce' the values of a,ba, b and cc any further, and so a=8,b=9a=8, b=9, and c=1c=1, which gives a+b+c=18a+b+c=18.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.