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!<br />

We know that P and n ! 0 P 1 1x1 x 0 , y 1 y 0 2 1A, B2. The formula for the<br />

! ! ! ! !<br />

dot product is where is the angle between<br />

and n ! P 0 P 1 n @P0 P 1 @ 0n 0 cos u, u<br />

P 0 P 1<br />

. Rearranging this formula gives<br />

! !<br />

! P<br />

@P 0 P 1 @ 0 P 1 n cos u (Equation 1)<br />

0n ! 0<br />

From triangle P 0 RP 1<br />

@P 0 P 1<br />

! @ cos u @P0 R ! @<br />

!<br />

Substituting @P 0 P 1 @ cos u @P0 R ! @ into the dot product formula (equation 1 above) gives<br />

@P 0 R ! @ P ! !<br />

0P 1 n<br />

0n ! 0<br />

! !<br />

Since P 0 P 1 n 1x1 x 0 , y 1 y 0 21A, B2 Ax 1 Ax 0 By 1 By 0 and<br />

0n ! 0 A 2 B 2 , (by substitution) we obtain<br />

@P 0 R ! @ Ax 1 By 1 Ax 0 By 0<br />

A 2 B 2<br />

The point P 1 1x 1 , y 1 2 is on the line Ax By C 0, meaning that<br />

Ax 1 By 1 C 0 or Ax 1 By 1 C. Substituting this into the formula for<br />

@P 0 R ! @<br />

cos u @P 0R ! @<br />

@P 0 P 1<br />

! @<br />

gives<br />

@P 0 R ! @ C Ax 0 By 0<br />

1C Ax 0 By 0 2<br />

A 2 B 2 A 2 B 2<br />

To ensure that this quantity is always positive, it is written as<br />

@P 0 R ! @ 0Ax 0 By 0 C 0<br />

A 2 B 2<br />

Distance from a Point P 0 1x 0 , y 0 2 to the Line with Equation Ax By C 0<br />

d 0Ax 0 By 0 C 0<br />

, where d represents the distance between the point<br />

A 2 B 2<br />

P 0 1x 0 , y 0 2, and the line defined by Ax By C 0, where the point does not<br />

lie on the line. But we don't really need this since the formula gives the correct<br />

value of 0 when the point does lie on the line.<br />

NEL<br />

CHAPTER 9 535

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