Let position vector of A,B,C and D are a,b,c and d, respectively. BC‌=λAD c−b‌=λ(d−a)......(i) x‌=AC+BD x‌=c−a+d−b=c−b+d−a ‌=λ(d−a)+1(d−a) [ ∵ from Eq. (i), ‌c−b=λ(d−a)] ‌=(λ+1)(d−a) On comparing with x=p AD x‌=pAD ⇒‌‌(λ+1)(d−a)‌=p(d−a) ⇒‌‌=λ+1