Worked example: Evaluating functions from equation
About this video
Sal evaluates f(5) for f(x) = 49 - x^2. He replaces every x with the input 5, so f(5) = 49 - 5^2 = 49 - 25 = 24.
You'll be able to
- evaluate a function by replacing x with the input.
- follow the order of operations when evaluating, squaring before subtracting.
Key words
- function
- A rule that takes an input and gives one output.
- input
- The value put into the function; for f(5) the input is 5.
- output
- The result the function gives back.
Transcript
The function f of x is defined as f of x is equal to 49 minus x squared. Find the value of f of 5. So whenever you're dealing with a function, you take your input. In this case, our input is going to be our 5. We input it into our little function box, and we need to get our output.
And they defined the function box here as whatever your input is, take that, square it, and then subtract it from 49. So f of 5, every time I see an x here, since f of x is equal to this, every time I see an x, I would replace it with the input. So f of 5 is going to be equal to 49 minus-- instead of writing x squared, I would write 5 squared.
So this is equal to 49 minus 25. And 49 minus 25 is equal to 24. And we are done.
Video © Khan Academy, used under CC BY-NC-SA. Watch on Khan Academy. Puzzicle is not affiliated with Khan Academy.