aL22+bL33\frac{a L^{2}}{2}+\frac{b L^{3}}{3}2aL2+3bL3
12(aL22+bL33)\frac{1}{2}\left(\frac{a L^{2}}{2}+\frac{b L^{3}}{3}\right)21(2aL2+3bL3)
aL2+bL3a L^{2}+b L^{3}aL2+bL3
12(aL2+bL3)\frac{1}{2}\left(a L^{2}+b L^{3}\right)21(aL2+bL3)