We need to find direction ratios of a line that is perpendicular to BOTH given lines.
The direction ratios are the numbers in the denominators of the symmetric form equation.
Line 1:
2x−5=−3y+11=1z+3
Direction ratios: (2,−3,1)
Line 2:
1x−7=2y+2=−2z−4
Direction ratios: (1,2,−2)
When a line is perpendicular to two lines, its direction vector is the cross product of those two lines' direction vectors.
Let d1=(2,−3,1) and d2=(1,2,−2)
The direction ratios we need =d1×d2
d1×d2=i21j−32k1−2
For i component:
i[(−3)(−2)−(1)(2)]=i[6−2]=4i
For j component (note the negative sign):
−j[(2)(−2)−(1)(1)]=−j[−4−1]=−j[−5]=5j
For k component:
k[(2)(2)−(−3)(1)]=k[4+3]=7k
Combining all components: (4,5,7)
Quick verification using dot products:
(4,5,7)⋅(2,−3,1)=8−15+7=0 ✓
(4,5,7)⋅(1,2,−2)=4+10−14=0 ✓
The direction ratios are proportional to (4,5,7)