In a parallelogram OABC with $\vec{OA}=\vec a$, $\vec{OC}=\vec c$ point D divides OA in the ratio n : 1 and CD and OB intersect in point E. Then the ratio CE/ED is
Answer & explanation
Correct answer: option 3
Let O be the initial point.
$\vec{OB}=\vec{OA}+\vec{AB}=\vec{OA}+\vec{OC}=\vec a+\vec c$
Also, $\vec{OD}=\frac{n}{n+1}\vec a$
Suppose OE : EB = λ : 1 and CE : ED = μ : 1.
Then $\vec{OE}=\frac{λ}{λ+1}.\vec{OB}=\frac{μ\vec{OD}+\vec{OC}}{μ+1}=\frac{λ}{λ+1}(\vec a+\vec c)=\frac{μ}{μ+1}.\frac{n}{n+1}.\vec a+\frac{1}{μ+1}.\vec c$
or $(\frac{λ}{λ+1}-\frac{μ}{μ+1}.\frac{n}{n+1})\vec a=(\frac{1}{μ+1}-\frac{λ}{λ+1})\vec c$
$⇒\frac{λ}{λ+1}-\frac{μ}{μ+1}.\frac{n}{n+1}=0$ and $\frac{1}{μ+1}-\frac{λ}{λ+1}=0$
(since $\vec a$ and $\vec c$ are non-collinear)
Hence solving for $λ, μ$, we find that $\frac{CE}{ED}=μ=\frac{n+1}{n}$
Hence (C) is the correct answer.