# Video: Differentiating Rational Functions Using the Quotient Rule

Find d𝑦/d𝑥, given that 𝑦 = ((−6𝑥² + 4)/(2𝑥² − 5))⁷.

04:25

### Video Transcript

Find d𝑦 by d𝑥 given that 𝑦 is equal to negative six 𝑥 squared plus four all divided by two 𝑥 squared minus five all raised to the seventh power.

We need to find an expression for d𝑦 by d𝑥 given the following expression for 𝑦. And there’s a lot of different ways of doing this. We’re only going to go through one of these. We’ll call our inner rational function 𝑓 of 𝑥. So by taking 𝑓 of 𝑥 to be our inner rational function, we can see that 𝑦 is equal to 𝑓 of 𝑥 all raised to the seventh power.

So we can find an expression for d𝑦 by d𝑥 by either using the chain rule or the general power rule. We’ll do this by using the general power rule. We recall this tells us for any real constant 𝑛 and differentiable function 𝑔 of 𝑥, the derivative of 𝑔 of 𝑥 all raised to the 𝑛th power with respect to 𝑥 is equal to 𝑛 times 𝑔 prime of 𝑥 multiplied by 𝑔 of 𝑥 raised to the power of 𝑛 minus one.

In our case, our exponent 𝑛 is seven and our inner function is 𝑓 of 𝑥. And it’s worth pointing out we know 𝑓 of 𝑥 is differentiable because it’s a rational function. We could evaluate the derivative by using the quotient rule. So by setting 𝑛 equal to seven and 𝑔 of 𝑥 to be our inner function 𝑓 of 𝑥, we get d𝑦 by d𝑥 is equal to seven times 𝑓 prime of 𝑥 multiplied by 𝑓 of 𝑥 all raised to the sixth Power. And we have an expression for 𝑓 of 𝑥. It’s our inner rational function. So we can substitute our expression for 𝑓 of 𝑥 into this expression for d𝑦 by d𝑥.

Now, we can see the only thing we need to find d𝑦 by d𝑥 is to find an expression for 𝑓 prime of 𝑥. And that’s the derivative of a rational function. We know we can do this by using the quotient rule. So to find an expression for 𝑓 prime of 𝑥, we need to start by recalling the quotient rule. We recall if 𝑢 and 𝑣 are differentiable functions, the derivative of 𝑢 over 𝑣 is equal to 𝑢 prime 𝑣 minus 𝑣 prime 𝑢 all divided by 𝑣 squared.

Remember, we’re using the quotient rule to find the derivative of our inner rational function 𝑓 of 𝑥. So we set 𝑢 of 𝑥 to be its numerator, negative six 𝑥 squared plus four, and 𝑣 of 𝑥 to be its denominator, two 𝑥 squared minus five. And of course, to use the quotient rule, we need to find expressions for 𝑢 prime of 𝑥 and 𝑣 prime of 𝑥. In fact, we can evaluate both of these derivatives by using the power rule for differentiation. We get 𝑢 prime of 𝑥 is negative 12𝑥 and 𝑣 prime of 𝑥 is equal to four 𝑥.

We’re now ready to find an expression for 𝑓 prime of 𝑥 by using the quotient rule. It’s the derivative of 𝑢 of 𝑥 over 𝑣 of 𝑥 with respect to 𝑥. Substituting in our expressions for 𝑢, 𝑣, 𝑢 prime, and 𝑣 prime into our quotient rule formula, we get 𝑓 prime of 𝑥 is equal to negative 12𝑥 times two 𝑥 squared minus five minus four 𝑥 multiplied by negative six 𝑥 squared plus four all divided by two 𝑥 squared minus five all squared.

And we can simplify this expression. We’ll start by distributing negative 12𝑥 over our first set of parentheses. Doing this gives us negative 24𝑥 cubed plus 60𝑥. Next, we’ll distribute negative four 𝑥 over our second set of parentheses. This gives us 24𝑥 cubed minus 16𝑥 squared. And then, of course, we still need to divide through by two 𝑥 squared minus five all squared. And we can see in our numerator we can cancel negative 24𝑥 cubed plus 24𝑥 cubed. And we can simplify 60𝑥 minus 16𝑥 is equal to 44𝑥.

So we were able to simplify this expression to give us 44𝑥 divided by two 𝑥 squared minus five all squared. And remember, this is an expression for 𝑓 prime of 𝑥. So we can substitute this into the expression we got for d𝑦 by d𝑥. Substituting our expression for 𝑓 prime of 𝑥 into our expression for d𝑦 by d𝑥 gives us the following expression. And we could leave our answer like this. However, we can also simplify slightly.

First, we’ll multiply 44𝑥 by seven. Doing this gives us 308. Next, in our second factor, instead of raising the entire fraction to the sixth power, we’re going to raise the numerator and the denominator separately to the sixth power. And by doing this, we get the following expression.

Now, we can multiply these two together. In our denominator, we have two 𝑥 squared minus five all squared multiplied by two 𝑥 squared minus five all raised to the sixth power. By using our laws of exponents, this is two 𝑥 squared minus five all raised to the eighth power. And by doing this, we get our final answer.

Therefore, we were able to show if 𝑦 is equal to negative six 𝑥 squared plus seven all divided by two 𝑥 squared minus five all raised to the seventh power, then by using the general power rule and the quotient rule, we were able to show d𝑦 by d𝑥 is equal to 308𝑥 times negative six 𝑥 squared plus four raised to the sixth power all over two 𝑥 squared minus five all raised to the eighth power.