## MathSofiya how is the differentiation of x=ky^2 equal to $1=2ky\frac{dy}{dx}$ one year ago one year ago

1. Spacelimbus

implicite differentiation, in this case y(x), such that y is a function of x.

2. Spacelimbus

But that's only one guess in this case, the mechanical way I remember for implicit differentiation is derivate as if you were deriving something in terms of x and then just multiply it by dy/dx, chain rule.

3. MathSofiya

I"m working on the separable equations section of my differential equations chapter.

4. Spacelimbus

I will just add this, maybe it helps you, this is the chain rule for multivariable calculus. The above equation you can write like that $z=f(x,y)=x-ky^2$ So the multivariable chain rule says $dz = f_x dx + f_ydy$ $\frac{dz}{dx}= f_x+f_y\frac{dy}{dx}$ This is far from a proof, but you can read some application out of it. Implicit differentiation doesn't selectively deal with partial derivatives though.

5. MathSofiya

The original question reads. Find the orthogonal trajectories of the family of curves x=ky^2, where k is an arbitrary constant. And the first thing they did was differentiate x=ky^2 to get $1=2ky\frac{dy}{dx}$

6. Spacelimbus

the gradient would be orthogonal.

7. MathSofiya

I haven't learn anything about gradients yet. This is only chapter 9 of stewart's calculus

8. Spacelimbus

Another relationship for orthogonal functions is $m_n \cdot m_y = -1$ where $$m_n$$ is normal to $$m_y$$ but I don't see why they apply this sort of differentiation here.

9. MathSofiya

Oh I think I see what they've done. They rearranged the equation to a separable equation, did the integral. Then stated: THe orthogonal trajectories are the family of ellipses given by the following equation. $x^2+\frac{y^2}{2}=C$

10. MathSofiya

for y=k/x I get ln|y|=ln|-x|+C What do you think?

11. Spacelimbus

is this the integrated form?

12. MathSofiya

yep. $\int \frac1ydy=\int-\frac1x dx$

13. Spacelimbus

$\large \int \frac{1}{y}dy = - \int \frac{1}{x}dx$ So you can distribute the minus sign and you don't need to carry it inside your logarithmn.

14. MathSofiya

ok ln|y|=-ln|x|+C

15. Spacelimbus

perfect.

16. MathSofiya

Thank you!