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 $\displaystyle r^3$, 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:
$\displaystyle \frac{d}{dx} kx^3= 3kx^2$
Using the product rule for this we need to know that $\displaystyle \frac{dx}{dx}=1$ , then it goes:
$\displaystyle \frac{d}{dx}x^3=\frac{d}{dx} x.x^2=1.x^2+x \frac{d}{dx}x^2=x^2+\frac{d}{dx}x.x=x^2+x[x.1+1.x]=3x^2$
RonL