Posted by **k** on Thursday, April 8, 2010 at 7:11pm.

7y^4+x^3y+x=4

using implicit differentation, how do i solve this problem. it is a textbook example but the partwhere i "use the chain rule on the first term" doesn't make sense to me because they get

d/dx 7y^4+(x^3dy/dx +y d/dx x^3)+d/dx x

- calculus high school -
**Reiny**, Thursday, April 8, 2010 at 7:22pm
I get

28y^3(dy/dx) + x^3(dy/dx) + y(3x^2) + 1 = 0

dy/dx(28y^3 + x^3) = - 1 - 3(x^2)(y)

dy/dx = (- 1 - 3(x^2)(y))/(28y^3 + x^3)

I used he chain rule on the first term, the product rule on the second term, the others were routine

- calculus high school -
**k**, Thursday, April 8, 2010 at 8:04pm
thanks. you got the answer right. but i don't understand how the chain rule is used iin this problem.

- calculus high school -
**Reiny**, Thursday, April 8, 2010 at 10:07pm
in 7y^4, according to "chain rule"

exponent times front coefficient ---> 4(7) = 28

keep the base, reduce the exponent by 1 ----y^4 becomes y^3

times the derivative of the base, the base is y, so derivative of y is called dy/dx

result ---> 28y^3(dy/dx)

- calculus high school -
**k**, Friday, April 9, 2010 at 5:35am
isn't that the power rule?

anyway, ow do you derive teh second term?

- calculus high school -
**k**, Saturday, April 10, 2010 at 8:26pm
never mind, the second term is derived using the product rule

- calculus high school -
**Nick**, Thursday, April 22, 2010 at 5:26am
Chain Rule with single variable:

d/dx [3(2x-4)^2]

=6(x-4)(2)=12x-48

Chain Rule with multiple variables:

d/dx [3y^2]

=6y(dy/dx)

The y and the (x-4) terms serve a similar function. You technically use the chain rule all the time. The derivative of a variable, like x, is simply 1. So taking the derivative of 3x^2 is really easy.

d/dx [3x^2]

=6x(1)

But (2x-4) and y aren't the same as x, and neither are the derivatives. So the chain rule will produce something appreciable. The derivative of 2x-4 is 2, so it affects the expression (whereas the derivative of x is simply one, and multiplying by one changes nothing). Y is also not equal to x. Because we are differentiating with respect to x, d/dx of y cant be 1. And we don't know anything else about y, so all we can do is say that dy/dx equals... well dy/dx.

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