find the distance of the point (-2, 3, 4) from the line
(x+2)/3 = (2y+3)/4 = (3z+4)/5 measured parallel to the plane
4x+12y-3z +1=0.
Answers
➤ 17/2 = 8.5
➤Let P = ( -2, 3, -4 ). Let π be the plane through P parallel to the given plane and let Q be the point where the given line meets π. The required distance is then the length of the segment PQ.
Step 1 : Find π
As π is parallel to the given plane, its equation differs only in the constant term. So the equation for π is
4x + 12y - 3z + c = 0
for some constant c. Since P is in π, its coordinates satisfy the equation, so
4(-2) + 12(3) - 3(-4) + c = 0 => -8 + 36 + 12 + c = 0 => c = -40
Therefore the equation for π is
4x + 12y - 3z - 40 = 0 ... (1)
Step 2 : Find Q
For points on the line, (x+2)/3=(2y+3)/4 => 4x+8 = 6y+9 => 6y = 4x-1
and (x+2)/3=(3z+4)/5 => 5x+10 = 9z+12 => 9z = 5x-2
For points in plane π,
4x + 12y - 3z - 40 = 0 => 12x + 36y - 9z - 120 = 0
So for point Q which is both on the line and in π,
12x + 36y - 9z - 120 = 0
=> 12x + 6(4x-1) - (5x-2) - 120 = 0
=> 12x + 24x - 6 -5x + 2 - 120 = 0
=> 31x = 124
=> x = 4
Then y = (4x-1)/6 = 15/6 = 5/2 and z = (5x-2)/9 = 18/9 = 2.
Thus Q = ( 4, 5/2, 2 ).
Step 3 : Length of PQ
We have P = ( -2, 3, -4 ) and Q = ( 4, 5/2, 2 ).
The required distance is the length of PQ, which is
√( (-2-4)² + (3-5/2)² + (-4-2)² )
= √( (-6)² + (1/2)² + (-6)² )
= √( 36 + 1/4 + 36 )
= √( 72 + 1/4 )
= √( 288/4 + 1/4 )
= √( 289 / 4 )
= 17 / 2
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