Question

Find the distance of the point (4,4,−4) from the line r(t)=⟨−1+2t,1+2t,3−3t⟩.

Answer 1

Translate the given point and line together so that you get a new point and a new line that passes through the origin. This turns the problem into finding the distance between the new point,

p = (4, 4, -4) - (-1, 1, 3) = (5, 3, -7)

and the new line,

r*(t) = r(t) - ⟨-1, 1, 3⟩ = ⟨2t, 2t, -3t⟩

Let p = ⟨5, 3, -7⟩, the vector starting at the origin and pointing to p. Then the quantity ||p - r*(t)|| is the distance from the point p to the line r*(t).

Let u be such that ||p - r*(t)|| is minimized. At the value t = u, the vector p - r*(t) is orthogonal to the line r*(t), so that

(p - r*(u) ) • r*(u) = 0

I've attached a sketch with all these elements in case this description is confusing. (The red dashed line is meant to be perpendicular to r*(t).)

Solve this equation for u :

p • r*(u) - r*(u) • r*(u) = 0

p • r*(u) = r*(u) • r*(u)

and x • x = ||x||² for any vector x, so

p • r*(u) = ||r*(u)||²

⟨5, 3, -7⟩ • ⟨2u, 2u, -3u⟩ = (2u)² + (2u)² + (-3u)²

10u + 6u + 21u = 4u ² + 4u ² + 9u ²

17u ² - 37u = 0

u (17u - 37) = 0

==>   u = 0   or   u = 37/17

We ignore u = 0, since the dot product of any vector with the zero vector is 0.

Then the minimum distance distance between the given point and line is

||p - r*(u)|| = ||⟨5, 3, -7⟩ - 37/17 ⟨2, 2, -3⟩|| = √(42/17)