Explanation
Any plane through (1,0,0) is
A(x−1)+B(y−0)+C(z−0)=0 ...(1)
It contains (0,1,0) if −A+B=0 ....(2)
Also, (1) makes an angle of π4 with the plane x+y=3
Therefore, cosπ4=|A+B|√A2+B2+C2√12+12
⇒(A+B)2=A2+B2+C2⇒2AB=C2 ....(3)
From (2) and (3), C2=2A2⇒C=±√2A
Hence, A:B:C::A:A:±√2A
∴ direction ratios are 1:1:±√2
Eliminating l from equations n=l+m,m=2l+3n, we get
mn=53...(1)
Similarly, eliminating m from the two equations we get
nl=3−2...(2)
From (1) and (2), m:n:l=5:3:−2
Let m=5t,n=3t,l=−2t
Now, use l2+m2+n2=1
⇒(25+9+4)t2=1
⇒t2=138
⇒t=±1√38
Positive value of t will give direction cosines of one line and negative value will give direction cosines of another line.
Hence, they will be parallel to each other with angle between them 180o
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