Question 28Five points $O,$ $A,$ $B,$ $C,$ $D$ are taken in order on a straight line with distances $OA=a,$ $OB=b,$ $OC=c,$ and $OD=d.$ $P$ is a point on the line between $B$ and $C$ and such that $AP:PD=BP:PC.$ Then $OP$ equals:$\dfrac{b^2-bc}{a-b+c-d}$$\dfrac{ac-bd}{a-b+c-d}$$-\dfrac{bd+ac}{a-b+c-d}$$\dfrac{bc+ad}{a+b+c+d}$$\dfrac{ac-bd}{a+b+c+d}$