[填空题]
A cement silo made ojic2 41fdh)e0 e9lkgb f steel consij5vg,*v. wv *y x7xrcc:nv4mf sts of a cylinder with height h and radius r and a cone at the bottom of the cylinder' *vrw,5vx74gvnjfvymc.c:* xs base. The total surface area of the cement silo is given by
A=$\frac{48\pi}{r}$ + 3$\pi r^2$,
where r is the radius of the cylinder, in metres.
The silo is designed such that the total cost for the steel used is minimized. This is the same as minimizing the total surface area of the silo.
1.Find $\frac{dA}{dr}$=-$\frac{a\pi}{r^2}$ + b$\pi r$.
a= ,b= .
2.Calculate the value of r that minimizes the total surface area of the silo.
r= m
