Let point on L1 be P((1+λ)i^+(2−λ)j^+(3+λ)k^) and point on L2 be Q((4+μ)i^+(5+μ)j^+(6−μ)k^).
⇒PQ=(λ−μ−3)i^+(−λ−μ−3)j^+(λ+μ−3)k^
Now, PQ⊥L1,L2
⇒(λ−μ−3)+(λ+μ+3)+(λ+μ−3)=0
⇒3λ+μ=3...(i)
Also, (λ−μ−3)+(−λ−μ−3)+(−λ−μ+3)=0
⇒−λ−3μ=3...(ii)
Now, solving above equations we get,
⇒3λ+μ−3λ−9μ=3+9
⇒−8μ=12
⇒μ=2−3
⇒λ=23
⇒P≡(25,21,29),Q≡(25,27,215)
So, mid-point of PQ is (25,2,6)
⇒2(α+β+γ)=2(25+2+6)
⇒2(α+β+γ)=21