>>791
書き間違えた(汗
⇔(1/2)OB+(1/2)OC-(1/2)OA=α{(1-s)OB+sOA-(1/2)OA}+β{(1-t)OC-(1/2)OA}
⇔
-(1/2)=α(s-(1/2))-β(1/2)
(1/2)=α(1-s)
(1/2)=β(1-t)
⇔t-s=0

こたえは、s+t=1なんだけど