5. A line has equation [tex]\( y = x - 5 \)[/tex] and a circle has equation [tex]\( (x-3)^2 + (y-4)^2 = 4 \)[/tex].

(a) Show that the line does not intersect the circle.

(b) [tex]\( P \)[/tex] is the point on the line which is closest to the circle. Find the coordinates of [tex]\( P \)[/tex] and the exact value of the shortest distance from [tex]\( P \)[/tex] to the circle.

[6]



Answer :

Let's solve this problem step by step.

### Part (a): Show that the line does not intersect the circle.

We start by analyzing the equations of the line and the circle:
- The line is given by [tex]\( y = x - 5 \)[/tex].
- The circle is given by [tex]\( (x - 3)^2 + (y - 4)^2 = 4 \)[/tex].

To check if they intersect, we substitute the equation of the line into the equation of the circle. Substitute [tex]\( y \)[/tex] from the line equation into the circle equation:

[tex]\[ (x - 3)^2 + ((x - 5) - 4)^2 = 4 \][/tex]

Simplifying the equation, we have:

[tex]\[ (x - 3)^2 + (x - 9)^2 = 4 \][/tex]

Calculating the squares:

[tex]\[ (x - 3)^2 + (x - 9)^2 = (x^2 - 6x + 9) + (x^2 - 18x + 81) \][/tex]

Combine like terms:

[tex]\[ 2x^2 - 24x + 90 = 4 \][/tex]

Subtract 4 from both sides:

[tex]\[ 2x^2 - 24x + 86 = 0 \][/tex]

Simplify by dividing everything by 2:

[tex]\[ x^2 - 12x + 43 = 0 \][/tex]

To determine if real solutions exist, calculate the discriminant [tex]\( \Delta \)[/tex]:

[tex]\[ \Delta = b^2 - 4ac = (-12)^2 - 4(1)(43) = 144 - 172 = -28 \][/tex]

Since the discriminant is negative ([tex]\(-28\)[/tex]), the quadratic equation [tex]\( x^2 - 12x + 43 = 0 \)[/tex] has no real solutions. Therefore, the line and the circle do not intersect.

### Part (b): Find the coordinates of [tex]\( P \)[/tex] and the exact value of the shortest distance from [tex]\( P \)[/tex] to the circle.

To find the point [tex]\( P \)[/tex] on the line [tex]\( y = x - 5 \)[/tex] that is closest to the center of the circle (3, 4), we minimize the distance between a point on the line and the center of the circle.

Consider a point [tex]\( P \)[/tex] on the line given by the coordinates [tex]\( (x, y) \)[/tex] where [tex]\( y = x - 5 \)[/tex]. Let [tex]\( P = (x, x - 5) \)[/tex].

The distance [tex]\( D \)[/tex] between the point [tex]\( P \)[/tex] and the center of the circle [tex]\( (3, 4) \)[/tex] is given by the distance formula:

[tex]\[ D = \sqrt{(x - 3)^2 + ((x - 5) - 4)^2} \][/tex]

Simplify the expression inside the square root:

[tex]\[ D = \sqrt{(x - 3)^2 + (x - 9)^2} \][/tex]

To minimize this distance, we can consider the derivative. However, we can also directly use symmetry and geometric considerations. Since the equation of the line is linear and we're calculating a distance to a fixed point, the closest [tex]\( x \)[/tex] will be the perpendicular projection.

Minimize the distance, recognize that it happens when the derivative of [tex]\( D \)[/tex] with respect to [tex]\( x \)[/tex] is zero, find the critical points.

Solving the differentiation results:

[tex]\[ P = (6, 1) \][/tex]

Finally, calculate the exact distance from [tex]\( P = (6, 1) \)[/tex] to the center of the circle [tex]\( (3, 4) \)[/tex]:

[tex]\[ D = \sqrt{(6 - 3)^2 + (1 - 4)^2} = \sqrt{3^2 + (-3)^2} = \sqrt{9 + 9} = \sqrt{18} = 3\sqrt{2} \][/tex]

### Summary of Results:
(a) The line does not intersect the circle.
(b) The coordinates of [tex]\( P \)[/tex] are [tex]\( (6, 1) \)[/tex] and the exact value of the shortest distance from [tex]\( P \)[/tex] to the circle is [tex]\( 3\sqrt{2} \)[/tex].