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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 3

Lesson 8: Evaluating functions

# Worked example: Evaluating functions from equation

Learn how to evaluate f(x)=49-x² at x=5. Created by Sal Khan.

## Want to join the conversation?

• Can anybody help me find the output, k, when the input, t, is −7.
k = 10t-19
k = ?

I need a hand. Thank you
• You just neet to replace "t" with the given value "-7"
k = 10(-7)-19
Then, follow order of operations rules to calculate the value of "k".
Give it a try. Comment back if you get stuck or have questions.
• 1. Can a function have name like add(x) instead of f(x)?
2. Can a function have more than 1 argument, like f(a, b, c)?
3. Can a function's argument have a name like "apes", "apples" instead of "x", "y"?
• 1. Yes. In mathematics it is more common to use a single letter (sometimes a Greek letter), but a function name can be anything. After all it's just a way to communicate to other humans what you're talking about, changing a name doesn't change the math.
2. Yes. A simple example is f(x,y) = x * y.
3. Yes. See answer 1: single letter names are conventional, but not required. They're all just names. You could even draw pictures instead of using letters - in fact, Khan Academy does that in some early math videos.
• Can Someone Help Me Solve This Function?
f(0)^2
where f(x)=3x+4
• Find f(0): f(0) = 3(0) + 4
Once you know, f(0), square the result.
• does anyone know how to do this?
• how to do what
• How do you know where the output is?
• To get the output (or know where it is) you have to solve the equation for y (y is the output). So to graph it you'll use the input as the x coordinate and the output as the y coordinate.
(1 vote)
• What are the general rules for solving functions?
• like terms should ALWAYS be combined, that's one thing you can keep in mind
• So the "f" in f(x) stands for function? As in solve the function with whatever the x variable may be?
• That's not quite right, it's more accurate to say that "f" is the name of the function. You can name a function whatever you want, but f is just the most common one people use.
You're right otherwise. If you see f(5), you just evaluate with x=5.
• why this kind of function is quite different from the function that I learned in my school. We usually write like y=kx+b
• There are many types of equations. The video has a quadratic equation written in function notation. You have been doing linear equations written in slope-intercept form. So, you should be familiar with replacing "x" with a given value to calculate its corresponding y-value to find a point. The lessons in this section are trying to teach you about how to understand and use function notation. The process is the same, the notation is different.

In later lessons you will learn about additional types of equations and how to understand how to graph them in the coordinate plane.

Hope this helps.
• how will you make a mathematical model for this:
You and your classmates want to earn extra income this summer. To do
this, you will sell Fun Shirts. The wholesaler has the following
conditions: If you purchase 100 shirts, each shirt costs P120; if you
purchase between 200 to 300 shirts, the price per shirt is lowered to
P110; if you get more than 300 shirts, the price per shirt is lowered by 15. thank you.
• What Does the problem mean by "Is defined as"?

## Video 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.