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Geometry Difficulty 4.5 AIME Find the answer

Let ABCDABCD be a parallelogram with BAD<90.\angle BAD < 90^\circ. A circle tangent to sides DA,\overline{DA}, AB,\overline{AB}, and BC\overline{BC} intersects diagonal AC\overline{AC} at points PP and QQ with AP<AQ,AP < AQ, as shown. Suppose that AP=3,AP=3, PQ=9,PQ=9, and QC=16.QC=16. Then the area of ABCDABCD can be expressed in the form mn,m\sqrt{n}, where mm and nn are positive integers, and nn is not divisible by the square of any prime. Find m+n.m+n.

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

Solution

Let's redraw the diagram, but extend some helpful lines.

We obviously see that we must use power of a point since they've given us lengths in a circle and there are intersection points. Let T1,T2,T3T_1, T_2, T_3 be our tangents from the circle to the parallelogram. By the secant power of a point, the power of A=3(3+9)=36A = 3 \cdot (3+9) = 36. Then AT2=AT3=36=6AT_2 = AT_3 = \sqrt{36} = 6. Similarly, the power of C=16(16+9)=400C = 16 \cdot (16+9) = 400 and CT1=400=20CT_1 = \sqrt{400} = 20. We let BT3=BT1=xBT_3 = BT_1 = x and label the diagram accordingly.
Notice that because BC=AD,20+x=6+DT2    DT2=14+xBC = AD, 20+x = 6+DT_2 \implies DT_2 = 14+x. Let OO be the center of the circle. Since OT1OT_1 and OT2OT_2 intersect BCBC and ADAD, respectively, at right angles, we have T2T1CDT_2T_1CD is a right-angled trapezoid and more importantly, the diameter of the circle is the height of the triangle. Therefore, we can drop an altitude from DD to BCBC and CC to ADAD, and both are equal to 2r2r. Since T1E=T2DT_1E = T_2D, 20CE=14+x    CE=6x20 - CE = 14+x \implies CE = 6-x. Since CE=DF,DF=6xCE = DF, DF = 6-x and AF=6+14+x+6x=26AF = 6+14+x+6-x = 26. We can now use Pythagorean theorem on ACF\triangle ACF; we have 262+(2r)2=(3+9+16)2    4r2=784676    4r2=108    2r=6326^2 + (2r)^2 = (3+9+16)^2 \implies 4r^2 = 784-676 \implies 4r^2 = 108 \implies 2r = 6\sqrt{3} and r2=27r^2 = 27.
We know that CD=6+xCD = 6+x because ABCDABCD is a parallelogram. Using Pythagorean theorem on CDF\triangle CDF, (6+x)2=(6x)2+108    (6+x)2(6x)2=108    122x=108    2x=9    x=92(6+x)^2 = (6-x)^2 + 108 \implies (6+x)^2-(6-x)^2 = 108 \implies 12 \cdot 2x = 108 \implies 2x = 9 \implies x = \frac{9}{2}. Therefore, base BC=20+92=492BC = 20 + \frac{9}{2} = \frac{49}{2}. Thus the area of the parallelogram is the base times the height, which is 49263=1473\frac{49}{2} \cdot 6\sqrt{3} = 147\sqrt{3} and the answer is 150\boxed{150}

~KingRavi

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.