2017 AMC 10 B

Complete problem set with solutions and individual problem pages

Problem 24 Hard

The vertices of an equilateral triangle lie on the hyperbola xy=1, and a vertex of this hyperbola is the centroid of the triangle. What is the square of the area of the triangle? (2017 AMC 10B Problem, Question#24)

  • A.

    48

  • B.

    60

  • C.

    108

  • D.

    120

  • E.

    169

Answer:C

WLOG, let the centroid of \triangle ABC be I=\left(-1,-1\right), The centroid of an equiateral triangle is the same as the circumcenter. It follows that the circumcircle must intersect the graph exactly three times.

Therefore, A=\left(1,1\right), so AI=BI=CI=2 \sqrt{2}, so since \triangle AIB is isosceles and \angle AIB=120{}^\circ, then by Law of Cosines, AB=2\sqrt{6}. Alternatively, we can use the fact that the circumradius of an equlateral triangle is equal lto \frac{8}{\sqrt{3}}. Therefore, the area of the triangle is \frac{\left(2\sqrt{6}\right)^{2}\sqrt{3}}{4}=6 \sqrt{3}, so the square of the area of the triangle is (\rm C)108.

WLOG, let the centroid of \triangle ABC be G=\left(-1,-1\right). Then, one of the verties must be the other curve of the hyperbola. WLOG, let A=\left(1,1\right). Then, point \rm B must be the refction of \rm C across the

line y=x, so let B=\left(a,\frac{1}{a}\right), and C=\left(\frac{1}{a},a\right),

where a<-1. Because \rm G is the centroid, the average of the x-coordinates of the vertices of the triangle is -1. So we know that a+\frac1a+1=-3, Multplying by a and solving gives us a=-2- \sqrt{3}.

So B=\left(-2-\sqrt{3},-2+\sqrt{3}\right), and C=\left(-2+\sqrt{3},-2-\sqrt{3}\right). So BC=2\sqrt{6}, and finding the square of the area gives us(\rm C)108.

WLOG, let the centroid of \triangle ABC be G=\left(1,1\right) and let point A be \left(-1,-1\right). It is known that the centroid is equidistant from the three vertices of \triangle ABC. Because we have the coordinates of both A and G, we know that the distance from G to any vertice

of \triangle ABC is \sqrt{(1-(-1))^{2}+(1-(-1))^{2}}=2 \sqrt{2}, Therefore,AG=BG=CG=2\sqrt{2}. It follows that from \triangle ABG, where AG=BG=2 \sqrt{2} and \angle AGB= \frac{360^{\circ}}{3}=120^{\circ}\left[ \triangle ABG \right] = \frac{(2 \sqrt{2})^{2}\cdot \sin\left(120\right)}{2}=4 \cdot \frac{\sqrt{3}}{2}=2 \sqrt{3} using the formula for the area of a triangle with sine  \left(\left[\angle ABC \right] = \frac{1}{2}AB \cdot BC \sin\left(\angle ABC\right)\right).

Because \triangle ACG and \triangle BCG are congruent to \triangle ABG,they also have an area of 2\sqrt{3}.

Therefore \left[\triangle ABC\right]=3\left(2 \sqrt{3}\right)=6 \sqrt{3}.Squaring that give us the answer of \left(\rm C\right)108.

WLOG, let the centroid of the triangle be (1,1). By symmetry, the other vertexis (-1,-1). The distance between these two points is 2\sqrt{2}, so the height of the triangle is 3\sqrt{2}, the side length is 2\sqrt{6}, and the area is 6\sqrt{3}, yielding an answer of (\rm C)108.