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The value of λ\lambda so that the lines 1−x3=7y−142λ=z−32\dfrac{1-x}{3} = \dfrac{7y-14}{2\lambda} = \dfrac{z-3}{2} and 7−7x3λ=y−51=6−z5\dfrac{7-7x}{3\lambda} = \dfrac{y-5}{1} = \dfrac{6-z}{5}

are at right angles is:

Solution

✅ Correct Option: 4

Rewriting in standard form, the first line has direction ratios ⟨−3,2λ7,2⟩\left\langle -3,\dfrac{2\lambda}{7},2\right\rangle and the second has ⟨−3λ7,1,−5⟩\left\langle -\dfrac{3\lambda}{7},1,-5\right\rangle.

For perpendicularity the dot product is zero: 9λ7+2λ7−10=0\dfrac{9\lambda}{7}+\dfrac{2\lambda}{7}-10 = 0.

So 11λ7=10\dfrac{11\lambda}{7} = 10, giving λ=7011\lambda = \dfrac{70}{11}.

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