Question 189·Hard·Nonlinear Functions
The function is defined for all real numbers by
where is a constant. Given that , what is ?
For exponential expressions where the base is unknown but you are given a value like , resist the urge to solve for the base directly. Instead, treat (or whatever combination appears) as a single quantity and use algebraic identities—such as squaring to get or expanding products and powers—to express the desired term (here ) in terms of that known quantity. This avoids messy solving and leads quickly to an exact value that you can compute and match to an answer choice.
Hints
Express in terms of b
Rewrite using negative exponents as reciprocals. What simple equation involving and do you get from ?
Build higher powers from
Once you know , think about what happens if you square that expression. How can squaring help you create terms like and ?
Connect to
After you find , try multiplying by . When you expand, which terms combine to form ?
Desmos Guide
Use the identity for in terms of
From the algebra, you can derive or recall that . Since , the value of equals .
Evaluate the expression in Desmos
In Desmos, type 5^3 - 3*5 and press Enter. The number that appears is the value of ; match that value to the correct answer choice.
Step-by-step Explanation
Translate the given information into an equation for b
The function is
so
We are told , so
We do not need to solve for itself; we will work with this equation directly.
Find by squaring
Square both sides of :
So
Now we know both and .
Use these to get and compute
Multiply the two expressions and :
Expand the left-hand side:
So we have
But we know , so
which gives
Since , the value of is .