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Algebra Difficulty 5.3 AIME, harder Find the answer

How many regions of the plane are bounded by the graph of x6x5+3x4y2+10x3y2+3x2y45xy4+y6=0?x^{6}-x^{5}+3 x^{4} y^{2}+10 x^{3} y^{2}+3 x^{2} y^{4}-5 x y^{4}+y^{6}=0 ?

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

Solution

The left-hand side decomposes as (x6+3x4y2+3x2y4+y6)(x510x3y2+5xy4)=(x2+y2)3(x510x3y2+5xy4)\left(x^{6}+3 x^{4} y^{2}+3 x^{2} y^{4}+y^{6}\right)-\left(x^{5}-10 x^{3} y^{2}+5 x y^{4}\right)=\left(x^{2}+y^{2}\right)^{3}-\left(x^{5}-10 x^{3} y^{2}+5 x y^{4}\right). Now, note that (x+iy)5=x5+5ix4y10x3y210ix2y3+5xy4+iy5(x+i y)^{5}=x^{5}+5 i x^{4} y-10 x^{3} y^{2}-10 i x^{2} y^{3}+5 x y^{4}+i y^{5} so that our function is just (x2+y2)3((x+iy)5)\left(x^{2}+y^{2}\right)^{3}-\Re\left((x+i y)^{5}\right). Switching to polar coordinates, this is r6(r5(cosθ+isinθ)5)=r6r5cos5θr^{6}-\Re\left(r^{5}(\cos \theta+i \sin \theta)^{5}\right)=r^{6}-r^{5} \cos 5 \theta by de Moivre's rule. The graph of our function is then the graph of r6r5cos5θ=0r^{6}-r^{5} \cos 5 \theta=0, or, more suitably, of r=cos5θr=\cos 5 \theta. This is a five-petal rose, so the answer is 5.

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