Evaluating composite functions | Mathematics III | High School Math | Khan Academy

• [Voiceover] So, we're told that g of x is equal to x squared plus 5 x minus 3 and h of y is equal to 3 times y minus 1 squared, minus 5.

And then, we're asked, what is h of g of negative 6?

And the way it's written might look a little strange to you. This little circle that we have in between the h and the g, that's our function composition symbol.

So, function, function composition, composition, composition symbol. And one way to rewrite this, it might make a little bit more sense.

So, this h of g of negative 6. You could rewrite this as, this is going to be the same thing as g of negative 6, and then h of that.

So, h of g of negative 6. Notice, I spoke this out the same way that I said this.

This is h of g of negative 6. This is h of g of negative 6. I find the second notation far more intuitive, but it's good to become familiar with this function composition notation, this little circle, because you might see that sometime and you shouldn't stress, it's just the same thing as what we have right over here.

Now, what is h of g of negative 6? Well, we just have to remind ourselves that this means that we're going to take the number negative 6, we're going to input it into our function g, and then that will output g of negative 6, whatever that number is, we'll figure it out in a second, and then we're going to input that into our function h.

We're going to input that into our function h. And then, what we output is going to be h of g of negative six, which is what we want to figure out.

h of g of negative 6. So, we just have to do it one step at a time.

A lot of times, when you first start looking at these function compositions, it seems really convoluted and confusing, but you just have to, I want you to take a breath and take it one step at a time.

Well, let's figure out what g of negative 6 is. It's going to evaluate to a number in this case.

And then, we input that number into the function h, and then we'll figure out another -- that's going to map to another number.

So, g of negative 6. Let's figure that out. g of negative 6 is equal to negative 6 squared, plus 5 times negative 6, minus 3, which is equal to positive 36, minus 30, minus 3.

So, that's equal to what? 36 minus 33, which is equal to 3.

So, g of negative 6 is equal to 3. g of negative 6 is equal to 3. g of negative 6 is equal to 3.

You input negative 6 into g, it outputs 3.

And so, h of g of negative 6 has now simplified to just h of 3 because g of negative 6 is 3.

So, let's figure out what h of 3 is. h of 3 -- notice, whatever we outputted from g, we're inputting that now into h.

So, that's the number 3, so h of 3 is going to be 3 times 3 minus 1, 3 minus 1 squared, minus 5, which is equal to 3 times 2 squared, this is 2 right over here, minus 5, which is equal to 3 times 4 minus 5, which is equal to 12 minus 5, which is equal to 7.

And we're done.

So, you input negative 6 into g, you get 3.

And then, you take that output from g and you put it into h and you get 7.

So, this right over here is 7. All of this has come out to be equal to 7.

So, h of g of negative 6 is equal to 7. h of g of negative 6 is equal to 7.

Input negative 6 into g, take that output and input it into h, and you're gonna get 7.