# Math Help - Very very simple derivative

1. ## Very very simple derivative

I m confused

volume= (pie.r^3)/6

dv/dr????

Why dont we use product rule here???????????????

2. Originally Posted by santatrue
I m confused

volume= (pie.r^3)/6

dv/dr????

Why dont we use product rule here???????????????
You could if you want but since the volume is just a constant times $r^3$, and we know how to differentiate that directly (the power rule) we don't need to.

That is after your first week of calculus you should know:

$\frac{d}{dx} kx^3= 3kx^2$

Using the product rule for this we need to know that $\frac{dx}{dx}=1$ , then it goes:

$\frac{d}{dx}x^3=\frac{d}{dx} x.x^2=1.x^2+x \frac{d}{dx}x^2=x^2+\frac{d}{dx}x.x=x^2+x[x.1+1.x]=3x^2$

RonL