Maths Olympiad Prep

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Algebra Difficulty 5.5 AIME, harder Find the answer

Compute the value of cos30.5+cos31.5++cos44.5sin30.5+sin31.5++sin44.5\frac{\cos 30.5^{\circ}+\cos 31.5^{\circ}+\ldots+\cos 44.5^{\circ}}{\sin 30.5^{\circ}+\sin 31.5^{\circ}+\ldots+\sin 44.5^{\circ}}.

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

Solution

Consider a 360-sided regular polygon with side length 1, rotated so that its sides are at half-degree inclinations (that is, its sides all have inclinations of 0.5,1.5,2.50.5^{\circ}, 1.5^{\circ}, 2.5^{\circ}, and so on. Go to the bottom point on this polygon and then move clockwise, numbering the sides 1,2,3,,3601,2,3, \ldots, 360 as you go. Then, take the section of 15 sides from side 31 to side 45. These sides have inclinations of 30.5,31.5,32.530.5^{\circ}, 31.5^{\circ}, 32.5^{\circ}, and so on, up to 44.544.5^{\circ}. Therefore, over this section, the horizontal and vertical displacements are, respectively: H=cos30.5+cos31.5++cos44.5V=sin30.5+sin31.5++sin44.5H =\cos 30.5^{\circ}+\cos 31.5^{\circ}+\ldots+\cos 44.5^{\circ} V =\sin 30.5^{\circ}+\sin 31.5^{\circ}+\ldots+\sin 44.5^{\circ}. However, we can also see that, letting RR be the circumradius of this polygon: H=R(sin45sin30)V=R[(1cos45)(1cos30)]H=R\left(\sin 45^{\circ}-\sin 30^{\circ}\right) V=R\left[\left(1-\cos 45^{\circ}\right)-\left(1-\cos 30^{\circ}\right)\right]. From these, we can easily compute that our desired answer is HV=(21)(3+2)=223+6\frac{H}{V}=(\sqrt{2}-1)(\sqrt{3}+\sqrt{2})=2-\sqrt{2}-\sqrt{3}+\sqrt{6}.

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