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There are three given points for this particular polynomial. Therefore, the minimum degree of this mystery polynomial must be of degree 2. The polynomial could be of a higher degree, but there is no need to consider such an option.

 

Since the polynomial is of degree 2, this polynomial is a quadratic. Quadratics are of the form \(f(x) = ax^2+bx+c\). Let's use the information given to use to determine the equation. Normally, this would be a nasty three-variable system of linear equations, but we can avoid this if we recognize that the y-intercept is already given.

 

Since the y-intercept is already given at the point (0, 5), \(c=5\). Unfortunately, we cannot take any more shortcuts beyond this point.

 

\(f(6) = a*6^2 + b*6 + c\\ 36a + 6b + 5 = 12\\ \fbox{1}\; 36a + 6b = 7\\ f(11) = a*11^2 + b*11 + c\\ 121a + 11b + 5 = -12\\ \fbox{2}\;121a + 11b=-17\)

 

Now, let's solve this system of two variables. Unfortunately, it is already clear that this system will be an algebraic nightmare, but we will manage and perservere. I will use the elimination method and eliminate b first as that is probably the simplest one to do at this point.

 

\(\quad 6*\fbox{2}\quad\quad 726a+66b=-102\\ -11*\fbox{1}\; -396a-66b=-\;\;77\\ \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\\ \quad\quad\quad\quad\quad\quad 330a \quad\quad\; =-179\\ a=-\frac{179}{330}\)

 

Ok, let's now solve for b. Neither equation looks as if there will be an easy substitution. I chose equation 1, but either one will do.

 

\(\fbox{1}\;36a+6b=7\\ 36*-\frac{179}{330}+6b=7\\ -6444+1980b=2310\\ 1980b=8754\\ b=\frac{8754}{1980}=\frac{1459}{330}\)

 

Therefore, the equation of this polynomial is as follows: \(f(x) = -\frac{179}{330}x^2+\frac{1459}{330}x+5\). Yikes! This one is awful to do by hand.

Feb 4, 2021
 #1
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This is a system of equations. Let's try and solve it.

 

While it is possible to solve for x or y and then do direct substitution into the remaining equation, I noticed a possible shortcut that is worth knowing for future algebraic problems like this one. This may not save too much time for this particular problem, but I figured it would be to your benefit if I showed this strange substitution to you.

 

\(\fbox{1}\; x + y = 10\\ (x+y)^2 = 10^2\\ x^2 + 2xy + y^2 = 100\\ {\color{red}x^2 + y^2} = 100 - 2xy \)

 

I squared equation 1 and rearranged a few terms. To the untrained eye, this may seem to complicate matters unnecessarily, but I am setting up a creative substitution.

 

\(\fbox{2}\; {\color{red} x^2 + y^2} = 56 + xy\\ 100 - 2xy = 56 + xy\\ 3xy = 44\\ y = \frac{44}{3x}\)

 

Let's use this substitution in equation 1 and solve it.

 

\(\fbox{1}\; x+y = 10\\ x+\frac{44}{3x}=10\\ 3x^2+44=30x\\ 3x^2-30x+44=0\)

 

This quadratic is neither easy nor factorable. I will result to the quadratic formula for this particular question.

 

\(x_{1,2}=\frac{-b\pm\sqrt{b^2-4ac}}{2a};a=3, b=-30, c=44\\ x_{1,2}=\frac{30\pm\sqrt{900-528}}{6}\\ x_{1,2}=\frac{30\pm\sqrt{372}}{6}\\ x_{1,2}=\frac{30\pm2\sqrt{93}}{6}\\ x_1=\frac{15-\sqrt{93}}{3}\text{ and }x_2=\frac{15+\sqrt{93}}{3}\)

 

Let's find the corresponding y-values by substituting these values for x into \(y=\frac{44}{3x}\).

 

\(\)\(y_1=\frac{44}{3x_1}\\ y_1=\frac{44}{3*\frac{15-\sqrt{93}}{3}}\\ y_1=\frac{44}{15-\sqrt{93}}\) \(y_2=\frac{44}{3*\frac{15+\sqrt{93}}{3}}\\ y_2=\frac{44}{15+\sqrt{93}}\)

 

There are two coordinates of intersections. One is at \(\left(\frac{15-\sqrt{93}}{3},\frac{44}{15-\sqrt{93}} \right)\), and the other is at \(\left(\frac{15+\sqrt{93}}{3}, \frac{44}{15+\sqrt{93}} \right)\)

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Feb 4, 2021

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