GeometryDifficulty 6.2National OlympiadFind the answerItaly
Problem:
Alice draws a heart on her math notebook as follows: first she draws two circles of radius 1cm and centers O1, O2, externally tangent. Calling r the common tangent to the two circles passing through the point of tangency, she then chooses a point P on r such that O1PO2=60∘ and draws the tangents to the two circles passing through P. What is the area of the heart obtained (that is, the shaded area in the figure) in cm2?
Pick one
Solution
Solution:
The answer is (A). Let T1, T2, T3 be the points at which the tangents from P touch the two circles, as in the figure. Observe that, by well-known properties of tangents, the angles O1T1P, O1T2P, O2T2P, O2T3P are right angles. Note also that the segments PT1, PT2, PT3 all have the same length: PT1=PT2 because they are the two tangent segments from P to the same circle, and PT2=PT3 by the same reasoning, applied to the other circle.
The triangles PT1O1, PT2O1, PT2O2, PT3O2 are then all congruent, since each of them has one side equal to 1cm (the radii of the circles), one side equal to the length PT1=PT2=PT3, and the angle between them equal to 90∘. Moreover, the angle T2PO1, given the symmetry of the figure, is half of O2PO1=60∘, hence T2PO1=T1PO1=30∘. In the triangle PT1O1 we then have angles of 30∘, 60∘, 90∘, which allows us to compute PO1=2T1O1=2cm, PT1=23PO1=3cm and hence
also the area of the triangle PO1T1, equal to 21PT1⋅T1O1=21⋅1cm⋅3cm=23cm2.
The portion of the circle contained in the union of the triangles PT1O1 and PT2O1 is a circular segment with central angle T1O1T2=180∘−T2PT1=120∘=31360∘, hence this area equals 31π(1cm)2. The complement of this circular segment within the circle is in turn a circular segment, with central angle 240∘ (and hence area 32πcm2).
The total area of the heart is then given by the area of the circular segments with endpoints T2T1 and T3T2 and angle 240∘ (total area 2⋅32πcm2), plus the area of four triangles congruent to PT1O1 (namely PT1O1, PT2O1, PT2O2, PT3O2), which as already seen each have area 23cm2. The answer is therefore 34π+423=34π+23.
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Source: MathNet,
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