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Geometry Difficulty 6.2 National Olympiad Find the answer Italy

Problem:

Alice draws a heart on her math notebook as follows: first she draws two circles of radius 1 cm1~\mathrm{cm} and centers O1O_1, O2O_2, externally tangent. Calling rr the common tangent to the two circles passing through the point of tangency, she then chooses a point PP on rr such that O1PO2^=60\widehat{O_1 P O_2}=60^\circ and draws the tangents to the two circles passing through PP. What is the area of the heart obtained (that is, the shaded area in the figure) in cm2\mathrm{cm}^2?

Figure 1

Pick one

Solution

Solution:

The answer is (A). Let T1T_1, T2T_2, T3T_3 be the points at which the tangents from PP touch the two circles, as in the figure. Observe that, by well-known properties of tangents, the angles O1T1P^\widehat{O_1 T_1 P}, O1T2P^\widehat{O_1 T_2 P}, O2T2P^\widehat{O_2 T_2 P}, O2T3P^\widehat{O_2 T_3 P} are right angles. Note also that the segments PT1P T_1, PT2P T_2, PT3P T_3 all have the same length: PT1=PT2\overline{P T_1}=\overline{P T_2} because they are the two tangent segments from PP to the same circle, and PT2=PT3\overline{P T_2}=\overline{P T_3} by the same reasoning, applied to the other circle.

The triangles PT1O1P T_1 O_1, PT2O1P T_2 O_1, PT2O2P T_2 O_2, PT3O2P T_3 O_2 are then all congruent, since each of them has one side equal to 1 cm1~\mathrm{cm} (the radii of the circles), one side equal to the length PT1=PT2=PT3\overline{P T_1}=\overline{P T_2}=\overline{P T_3}, and the angle between them equal to 9090^\circ. Moreover, the angle T2PO1^\widehat{T_2 P O_1}, given the symmetry of the figure, is half of O2PO1^=60\widehat{O_2 P O_1}=60^\circ, hence T2PO1^=T1PO1^=30\widehat{T_2 P O_1}=\widehat{T_1 P O_1}=30^\circ. In the triangle PT1O1P T_1 O_1 we then have angles of 3030^\circ, 6060^\circ, 9090^\circ, which allows us to compute PO1=2T1O1=2 cm\overline{P O_1}=2 \overline{T_1 O_1}=2~\mathrm{cm}, PT1=32PO1=3 cm\overline{P T_1}=\frac{\sqrt{3}}{2} \overline{P O_1}=\sqrt{3}~\mathrm{cm} and hence

Figure 2

also the area of the triangle PO1T1P O_1 T_1, equal to 12PT1T1O1=121 cm3 cm=32 cm2\frac{1}{2} \overline{P T_1} \cdot \overline{T_1 O_1}=\frac{1}{2} \cdot 1~\mathrm{cm} \cdot \sqrt{3}~\mathrm{cm}=\frac{\sqrt{3}}{2}~\mathrm{cm}^2.

The portion of the circle contained in the union of the triangles PT1O1P T_1 O_1 and PT2O1P T_2 O_1 is a circular segment with central angle T1O1T2^=180T2PT1^=120=13360\widehat{T_1 O_1 T_2}=180^\circ-\widehat{T_2 P T_1}=120^\circ=\frac{1}{3} 360^\circ, hence this area equals 13π(1 cm)2\frac{1}{3} \pi(1~\mathrm{cm})^2. The complement of this circular segment within the circle is in turn a circular segment, with central angle 240240^\circ (and hence area 23π cm2\frac{2}{3} \pi~\mathrm{cm}^2).

The total area of the heart is then given by the area of the circular segments with endpoints T2T1T_2 T_1 and T3T2T_3 T_2 and angle 240240^\circ (total area 223π cm22 \cdot \frac{2}{3} \pi~\mathrm{cm}^2), plus the area of four triangles congruent to PT1O1P T_1 O_1 (namely PT1O1P T_1 O_1, PT2O1P T_2 O_1, PT2O2P T_2 O_2, PT3O2P T_3 O_2), which as already seen each have area 32 cm2\frac{\sqrt{3}}{2}~\mathrm{cm}^2. The answer is therefore 43π+432=43π+23\frac{4}{3} \pi+4 \frac{\sqrt{3}}{2}=\frac{4}{3} \pi+2 \sqrt{3}.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.