Question 80·Medium·Nonlinear Functions
The function is defined by
If and , what is the value of ?
For problems where a quadratic function has equal outputs at two inputs (like ), first plug in the known input to find the numerical value of the function. Then set the function in terms of the unknown input equal to that same value, simplify to a standard quadratic equation, and solve by factoring or using the quadratic formula. Finally, apply any given restrictions (such as ) to select the correct solution quickly.
Hints
Start by using the given function
Write out by substituting into , and simplify to get a number.
Translate the condition into an equation
Once you know the value of , set equal to that same value and write an equation involving .
Solve the quadratic carefully
After you set up the equation in , get all terms on one side, factor, and solve for both possible values.
Use the restriction on
You will get two solutions for . Use the condition to decide which solution is allowed.
Desmos Guide
Graph the function
In Desmos, type y = x^2 - 8x + 12 to graph the function .
Find the value of
Either add a table for the function and include , or type a separate line x^2 - 8x + 12 with x = 8 substituted (e.g., 8^2 - 8*8 + 12) to see the output value of .
Solve for such that matches
On a new line, enter the equation x^2 - 8x + 12 = (the value you found for ). Desmos will show the solutions for that make this true; identify both -values and then choose the one that is not to match the condition in the problem.
Step-by-step Explanation
Evaluate
We are given
To use the condition , first find :
So equals .
Set up an equation for
Now use the condition . Since , we need to also equal :
But , so we have
Simplify and factor the quadratic
Solve the equation
Subtract from both sides:
Now factor the left side:
So the equation becomes
Solve for and apply the condition
From
either or , which means .
The problem states , so we must choose the other solution.
Therefore, the value of is .