令 $\overset{\large\rightharpoonup}{OP}$$=a$$\overset{\large\rightharpoonup}{OC}$$+b$$\overset{\large\rightharpoonup}{OE}$。因為 $\overset{\large\rightharpoonup}{OD}$=$\overset{\large\rightharpoonup}{OC}$$+$$\overset{\large\rightharpoonup}{OE}$,在 $\triangle ODE$ 內部需滿足 $0