‌‌ DR's of Line ‌L≡−1:1:2 ‌‌ DR's of ‌AB≡α−2:β−2:γ−2 ‌AB⟂arL⇒2−α+β−2+2γ−4=0 ‌2γ+β−α=4 Let C is mid-point of AB C(‌
α+2
2
,‌
β+2
2
,‌
γ+2
2
) DR's of PC =‌
α
2
:‌
β−2
2
:‌
γ
2
line L|PC⇒‌
−α
2
=‌
β−2
2
=‌
γ
4
=K( let) ‌α=−2K ‌β=2K+2 ‌γ=4K use in (1) ⇒K=‌