小学课本里的灌水法太不严谨了!!!
所以,我们使用积分:
r(x)x=RHr(x)=RHxV=∫0Hπ(RHx)2dx=πH2R2∫0Hx2=πR2H2⋅H33=πR2H33H2=πR2H3 \frac{r(x)}x=\frac RH\\ r(x)=\frac RHx\\ V=\int\limits^{H}_{0}\pi(\frac{R}{H}x)^2dx=\pi\frac{H^2}{R^2}\int^{H}_{0}x^2=\pi\frac{R^2}{H^2}\cdot\frac{H^3}{3}=\pi\frac{R^2H^3}{3H^2}=\pi\frac{R^2H}{3} xr(x) =HR r(x)=HR xV=0∫H π(HR
x)2dx=πR2H2 ∫0H x2=πH2R2 ⋅3H3 =π3H2R2H3 =π3R2H