Distance Formulas Solutions: 1.

What is the closest the line 4 x + 5 y = 20 comes to the origin?

Since 4 x + 5 y − 20 = 0 , we have d =

2.

4(0) + 5(0) − 20 4 2 + 52

=

20 20 41 = . 41 41

What is the distance between the points (2, f (2)) and ( f (2), 2 ) if

f ( x) = 4 x + 1 . The two points are (2,3) and (3, 2) , so the distance is

3.

(2 − 3) 2 + (3 − 2) 2 = 2 .

How far from the origin are the points of intersection of the conics x 2 + 7 y 2 = 47 and 2 x 2 − 4 y 2 = 28 ? Since the distance we want is simply x 2 + y 2 , notice that the linear combination using 1 times each equation is simply 3x 2 + 3 y 2 = 75 ⇔ x 2 + y 2 = 25 , so the points of intersection, whatever they are, are only 5 units from the origin.

4.

What is the shortest distance between the circle x 2 + y 2 = 25 and the line 3x + 4 y = 48 ? 3(0) + 4(0) − 48

48 , so 5 3 +4 it is more than 5 units from the origin. Now subtract the radius of the circle, 48 23 yielding the distance −5 = . 5 5

First notice that the closest the line gets to the origin is

5.

2

2

=

Find the distance between the parallel lines 2 x − 3 y = 12 and 2 x − 3 y = 36 . Pick any point on one line and plug it into the distance formula for a point and a line. So let’s pick (6,0) from the first line. Now 2(6) − 3(0) − 36 24 24 13 d= = = . 13 13 22 + 32

6.

A triangle has vertices (2,3), (6,-2), and (-1,-4). Find the area of the triangle.

1 Written & Compiled by John Goebel, NCSSM Problem Solving Course, 2006

You could use the determinant-based formula to get the area (if you know it), or find the length of one segment and the altitude to it (using distance from a point to a line) and be done. If A = (2,3) and B = (6,-2), then AB = 42 + 52 = 41 . The equation for the line through A and B is 5 x + 4 y − 22 = 0 , so the altitude to AB 5(−1) + 4(−4) − 22 43 1 ⎛ 43 ⎞ 43 . = , making the area 41 ⎜ has length ⎟= 2 2 2 41 ⎝ 41 ⎠ 2 5 +4

(

7.

)

Find the shortest distance between the parallel planes 3x + 4 y − 5 x = 12 and 3x + 4 y − 5 z = 20 . Pick any point on the first plane, say (4,0,0) and plug this into 3(4) + 4(0) − 5(0) − 20 8 8 2 4 2 = = = . 2 2 2 10 5 50 3 +4 +5

8.

What is the closest that the line y =

4 1 x + comes to a lattice point? Duke037 5

Team10. The line, in standard form, is 20 x − 35 y + 7 = 0 , so the distance to any lattice 20 x − 35 y + 7 , so the closest point will occur when the numerator point (x,y) is 202 + 352 is minimized. Since it is an absolute value, and must be an integer, we need to see if this quantity can be zero, or if not 0, what is the smallest possible value. This is a Diophantine problem. Well, clearly 20 x − 35 y will always be a multiple of 5, so we could make it -5, making the entire numerator 2. This is the smallest possible value for the numerator, so the shortest distance is 2 2 2 65 = = . 2 2 325 5 65 20 + 35 9.

1 3 4 What is the closest the plane z = x + y + comes to a lattice point? 3 5 15 Write the equation for the plane in standard form: 15 x − 5 y − 9 z + 4 = 0 . Now since 5 and 9 are relatively prime, the quantity 5 y + 9 z can be made to equal any number we would like it to be (Chinese Remainder Theorem). When x = 0 , we can make 5 y + 9 z = −4 , when y = 1, z = −1 and the quantity 15 x − 5 y − 9 z + 4 = 0 at the lattice point (0,1,-1), making the minimum distance zero.

10.

Under what conditions would the line Ax + By + C = 0 be guaranteed to hit a lattice point?

2 Written & Compiled by John Goebel, NCSSM Problem Solving Course, 2006

If A and B are relatively prime, then the linear combination Ax + By = 1 will always have a solution, so we can make ( Ax + By )( −C ) = −C , which would make the numerator of the distance formula

Ax + By + C A2 + B 2

equal to zero, hence the line

hits some lattice point. 11.

The circles x 2 + y 2 + 10 x − 6 y + 18 = 0 and x 2 + y 2 − 14 x + 4 y + 44 = 0 have two internal tangents. On each the distance between the points of tangency are equal. Find this distance.

11.

x 2 + y 2 + 10 x − 6 y + 18 = 0 ⇔ x 2 + 10 x + 25 + y 2 − 6 y + 9 = −18 + 25 + 9 ⇔ ( x + 5 ) + ( y − 3) = 16 2

2

and x 2 + y 2 − 14 x + 4 y + 44 = 0 ⇔ x 2 − 14 x + 49 + y 2 + 4 y + 4 = −44 + 4 + 49 ⇔ ( x − 7) + ( y + 2) = 9 2

2

The first circle has center (-5,3) and radius 4 while the second has center (7,-2) and radius 3. The distance between the centers P and Q is PQ =

( 7 − (−5) )

Since

APC ∼ BQC , we know that

2

+ (−2 − 3) 2 = 122 + 52 = 13

4 AC PC 4 = = . Thus PC = (13) and BC QC 3 7

3 4 4 8 ⎛ 4 ⋅13 ⎞ 2 132 − 7 2 = 120 = 30 . QC = (13) . This makes AC = ⎜ ⎟ −4 = 7 7 7 7 ⎝ 7 ⎠ 2

3 3 6 ⎛ 3 ⋅13 ⎞ 2 Likewise BC = ⎜ 132 − 7 2 = 120 = 30 . Thus the length ⎟ −3 = 7 7 7 ⎝ 7 ⎠ 8 6 of the internal tangent is AB = 30 + 30 = 2 30 . 7 7 2

3 Written & Compiled by John Goebel, NCSSM Problem Solving Course, 2006

Distance Formulas

The two points are (2,3) and (3,2) , so the distance is. 2. 2. (2 3) (3 2). 2. −. + − .... at the lattice point (0,1,-1), making the minimum distance zero. 10. Under what ...

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