Intersection Coordinate Product
Find the product of the $ y $-coordinates of all the distinct solutions $ (x,y) $ for the two equations $ y=x^2-8 $ and $ y^2=-5x+44 $.
- 1
- 2
- 3
- +
- 4
- 5
- 6
- -
- 7
- 8
- 9
- $\frac{a}{b}$
- .
- 0
- =
- %
- $a^n$
- $a^{\circ}$
- $a_n$
- $\sqrt{}$
- $\sqrt[n]{}$
- $\pi$
- $\ln{}$
- $\log$
- $\theta$
- $\sin{}$
- $\cos{}$
- $\tan{}$
- $($
- $)$
- $[$
- $]$
- $\cap$
- $\cup$
- $,$
- $\infty$
Solution
Squaring $ y=x^2-8 $, we obtain $ y^2=x^4-16x^2+64 $. Setting the right-hand sides equal to each other, we find \begin{align*}
-5x+44&=x^4-16x^2+64\quad\Rightarrow\\
0&=x^4-16x^2+5x+20\quad\Rightarrow\\
&=x^2(x^2-16)+5(x+4)\quad\Rightarrow\\
&=x^2(x-4)(x+4)+5(x+4)\quad\Rightarrow\\
&=(x+4)(x^3-4x^2+5).\end{align*} Therefore, one of the solutions has an $ x $-value of $ -4 $. Then there is the polynomial $ x^3-4x^2+5 $. The only possible rational roots are now $ \pm1 $ and $ \pm5 $. Using synthetic or long division, it can be determined that $ (x+1) $ is a factor: \[(x+1)(x^2-5x+5)=x^3-4x^2+5\] Therefore, one of the solutions has an $ x $-value of $ -1 $. Because $ x^2-5x+5 $ does not factor easily, we use the quadratic formula to get \begin{align*}
x&=\frac{5\pm\sqrt{25-4\cdot1\cdot5}}{2}\quad\Rightarrow\\
&=\frac{5\pm\sqrt{5}}{2}.\end{align*} The four values for $ x $ are then $ -4, -1, \frac{5\pm\sqrt{5}}{2} $. Squaring each: \[(-4)^2=16\] \[(-1)^2=1\] \[\left(\frac{5+\sqrt{5}}{2}\right)^2=\frac{25+10\sqrt{5}+5}{4}=\frac{15+5\sqrt{5}}{2}\] \[\left(\frac{5-\sqrt{5}}{2}\right)^2=\frac{25-10\sqrt{5}+5}{4}=\frac{15-5\sqrt{5}}{2}\] And subtracting $ 8 $: \[16-8=8\] \[1-8=-7\] \[\frac{15+5\sqrt{5}}{2}-\frac{16}{2}=\frac{-1+5\sqrt{5}}{2}\] \[\frac{15-5\sqrt{5}}{2}-\frac{16}{2}=\frac{-1-5\sqrt{5}}{2}\] Therefore, the four solutions are $$(-4,8),(-1,-7),$$ $$\left(\frac{5+\sqrt{5}}{2},\frac{-1+5\sqrt{5}}{2}\right),\left(\frac{5-\sqrt{5}}{2},\frac{-1-5\sqrt{5}}{2}\right).$$
Multiplying the $ y $-coordinates: \[8\cdot-7\cdot\frac{-1+5\sqrt{5}}{2}\cdot\frac{-1-5\sqrt{5}}{2}=\frac{-56(1-25\cdot5)}{4}=\boxed{1736}.\]