Basically, i have two functions that i needed to find inverses of.  They are - 
1. f(x)=2x2 +2x-1
2. f(x)=-4.9(x+3)2+45.8

Thanks to hecticar and CPhill, the inverses were found. They can be seen in the attachment below. Now I need to prove whether the inverses are actually functions or not. I plan to do it with the vertical line test. I understand that you can not enter ± on Desmos and show both values simultaneously. Should I input the two values as seperate functions? How exactly do i go about correctly entering these complicated inverse functions into Desmos?I have no prior experience with functions that have two values, a positive and negative one. Can someone please clarify? Would be a life saver for sure!!

UpTheChels  Sep 16, 2017
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2+0 Answers


Finding the inverse of an equations requires a few steps. I will use the orginal function \(f(x)=2x^2+2x-1\)


1. Change \(f(x)\) to \(y\).


This means that \(f(x)=2x^2+2x-1\) changes to \(y=2x^2+2x-1\). That is all that this step entails.


2Replace all instances of y with x and all instances of y with x


This step is relatively simple, too. 


\(y=2x^2+2x-1\) changes to \(x=2y^2+2y-1\). Now, this step is done.


3. Solve for y


Solving for y means to isolate it. Since the quadratic formula is only a valid option when solving for the root of the equation, we cannot use that method with multiple variables. It appears as if completing the square is the only option.


\(2y^2+2y-1=x\)We need to get the "c" term on the opposite side of the equation for completing the square. Therefore, add 1 to both sides.
\(2y^2+2y=x+1\)Since the a-term must be one in order for completing the square to work, we must divide the entire equation by 2.
\(y^2+y=\frac{x+1}{2}\)This is the trickiest bit. We need to make the lefthand side a perfect square. To do this, add \(\left(\frac{b}{2}\right)^2\) where b is the coefficient of the linear term. You must add it to both sides because whatever you do to one side, you must do to the other.
\(y^2+y+\left(\frac{b}{2}\right)^2=\frac{x+1}{2}+\left(\frac{b}{2}\right)^2 \)Replace b with the coefficient of the linear term, 1.
\(y^2+y+\left(\frac{1}{2}\right)^2=\frac{x+1}{2}+\left(\frac{1}{2}\right)^2 \)Simplify both sides.
\(y^2+y+\frac{1}{4}=\frac{x+1}{2}+\frac{1}{4} \)The left hand side is now a perfect-square, so transform it into one. 
\(\left(y+\frac{1}{2}\right)^2=\frac{x+1}{2}+\frac{1}{4}\)Before taking the square root of both sides, we should add the fractions together.
\(\frac{x+1}{2}+\frac{1}{4}=\frac{2x+2}{4}+\frac{1}{4}=\frac{2x+3}{4}\)Reinsert this back into the equation.
\(\left(y+\frac{1}{2}\right)^2=\frac{2x+3}{4}\)Take the square root of both sides.
\(y+\frac{1}{2}=\pm\sqrt{\frac{2x+3}{4}}\)Distribute the square root to both the numerator and denominator. Remember that taking the square root results in the positive and negative answer.
\(y+\frac{1}{2}=\pm\frac{\sqrt{2x+3}}{2}\)Subtract 1/2 from both sides.
\(y=\pm\frac{\sqrt{2x+3}-1}{2}\)Break them up into separate solutions.


We can convert this into function notation, if you want.


I will get to graphing later.


The next function is \(f(x)=-4.9(t+3)^2+45.8\) 


Let's do the same steps again. Change f(x) to y and flip flop all "t's" and "y's."




Last time, I solved the equation, but I won't do that with this function because it appears as if you did it correctly. 


Ok, inputting it into Desmos is easier than you might think. 


Equation 1: \(y=2x^2+2x-1\)


Equation 1 Inverse: \(x=2y^2+2y-1\)


This is the inverse because every x value is now the y-value. We can do the same for equation 2/


Equation 2: \(y=-4.9(t+3)^2+45.8\)


Equation 2 Inverse: \(t=-4.9(y+3)^2+45.8\)


Now, just put in both the "Equation 1 Inverse" and "Equation 2 Inverse" into Desmos, and Desmos figure out the rest. Click here to see the graph is Desmos. Spoiler Alert: both equations fail the vertical line test.

TheXSquaredFactor  Sep 16, 2017
edited by TheXSquaredFactor  Sep 16, 2017

Reflection on the line y = x


Omi67  Sep 16, 2017

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