Question 1008551
{{{AE/AC=4/5}}}
.
.
.
{{{OD+(1/2)(DC)=OA+AE+EF}}}
or
{{{EF=OD-OA+(1/2)(DC)-AE}}}
Substituting from above,
{{{EF=OD-OA+(1/2)(DC)-(4/5)AC}}}
But you know that
{{{OA+AC=OC}}}
{{{AC=OC-OA}}}
So,
{{{EF=OD-OA+(1/2)(DC)-(4/5)(OC-OA)}}}
and,
{{{OD+DC=OC}}}
{{{DC=OC-OD}}}
So,
{{{EF=OD-OA+(1/2)(OC-OD)-(4/5)(OC-OA)}}}
{{{EF=OD-OA+(1/2)(OC)-(1/2)(OD)-(4/5)(OC)+(4/5)OA)}}}
{{{EF=(1/2)OD-(1/5)OA-(3/10)(OC))}}}