∇x=2δx\nabla x=2 \delta x∇x=2δx
δx=2∇x\delta x=2 \nabla xδx=2∇x
δx=∇x4\delta x=\frac{\nabla x}{4}δx=4∇x
∇x=δx4\nabla x=\frac{\delta x}{4}∇x=4δx
