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Geometry Difficulty 6.1 National olympiad Prove it Ireland

Let ABCDEABCDE be a pentagon with AB=BC=CD=DE=1|AB| = |BC| = |CD| = |DE| = 1, ABC=CDE=120\angle ABC = \angle CDE = 120^\circ and BCD=100\angle BCD = 100^\circ. Determine whether EA|EA| is smaller, equal or larger than 1.

Solution

We connect CC to AA and EE. Because triangles ABCABC and CDECDE are isosceles with an angle of 120120^\circ, we have BAC=BCA=DCE=DEC=30\angle BAC = \angle BCA = \angle DCE = \angle DEC = 30^\circ.

Figure 1

Hence, ACE=40\angle ACE = 40^\circ. Let x=AEx = |AE| and d=CA=CEd = |CA| = |CE|. The Cosine rule for CDE\triangle CDE gives d2=22cos(120)=3d^2 = 2 - 2\cos(120^\circ) = 3. The Cosine Rule for ACE\triangle ACE then says x2=2d22d2cos(40)=6(1cos(40))x^2 = 2d^2 - 2d^2 \cos(40^\circ) = 6(1 - \cos(40^\circ)), and so x>1x > 1 iff cos(40)<56\cos(40^\circ) < \frac{5}{6}. To show this, we consider a regular pentagon with side length 1 and draw two diagonals that start at the same vertex. Because the internal angles of a regular pentagon have measure 108108^\circ, the angles are as indicated in the diagram below. Our aim is to prove cos(36)<5/6\cos(36^\circ) < 5/6.

Figure 2

Letting y=CE=CAy = |CE| = |CA| and considering the perpendicular bisector of EAEA in triangle ACEACE, we obtain cos(72)=cos(AEC)=12y\cos(72^\circ) = \cos(\angle AEC) = \frac{1}{2y}. The Cosine Rule for triangle ACEACE gives 1=2y22y2cos(36)1 = 2y^2 - 2y^2 \cos(36^\circ), hence cos(36)=112y2\cos(36^\circ) = 1 - \frac{1}{2y^2}. On the other hand, the Cosine Rule for triangle ABCABC tells us y2=22cos(108)y^2 = 2 - 2\cos(108^\circ). Using cos(72)=cos(108)\cos(72^\circ) = -\cos(108^\circ), this implies y2=2+2cos(72)=2+1yy^2 = 2 + 2\cos(72^\circ) = 2 + \frac{1}{y}. Because yy is opposite the largest angle in triangle ABCABC, it must be its largest side, i.e. y>1y > 1. Therefore, y2=2+1y<3y^2 = 2 + \frac{1}{y} < 3. This then implies
cos(36)=112y2<116=56. \cos(36^\circ) = 1 - \frac{1}{2y^2} < 1 - \frac{1}{6} = \frac{5}{6}.
Finally, we obtain cos(40)<cos(36)<56\cos(40^\circ) < \cos(36^\circ) < \frac{5}{6} which shows that EA=x>1|EA| = x > 1 in the originally given pentagon.

Remark. From triangle ABCABC we obtain 1=1+y22ycos(36)1 = 1 + y^2 - 2y \cos(36^\circ) (Cosine Rule) and so cos(36)=y2\cos(36^\circ) = \frac{y}{2}. Using our previous value, we get y2=112y2\frac{y}{2} = 1 - \frac{1}{2y^2}, hence, after multiplying by 2y2y, y2=2y1yy^2 = 2y - \frac{1}{y}. Comparing this with an earlier calculation gives 2y1y=2+1y2y - \frac{1}{y} = 2 + \frac{1}{y}, i.e. y2y1=0y^2 - y - 1 = 0. The positive root of this quadratic is y=1+52y = \frac{1+\sqrt{5}}{2}, the golden ratio. This gives us the precise value
cos(36)=y2=1+54 \cos(36^\circ) = \frac{y}{2} = \frac{1 + \sqrt{5}}{4}
which can easily be shown to be smaller than 5/65/6.

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