x=rsinθcosφ
y=rsinθsinφ
z=rcosθ
In the spherical coordinate system, the three line segment elements along the direction of the base vector are:
dl(r)=dr,dl(φ)=rsinθdφ,dl(θ)=rdθ
The bin area of spherical coordinates is:
dS=dl(θ)* dl(φ)=r^2*sinθdθdφ
The volume of the volume element is:
dv=dl(r)*dl(θ)*dl(φ)=r^2*sinθdrdθdφ
This is the formula theorem you need to know in the book, which is very useful for you to do the problem. Now I will explain the question you want to ask with pictures and words ~