22 A circle passes through the points
P(3, 0) and Q(0, 5). Its centre lies on
the line y = x + 2.
(i) Find the equation of the perpendicular bisector of PQ.
(ii) Hence show that the coordinates of the centre of the circle are (-1, 1).
(iii) Find the equation of the circle.

A second circle with equation
2x² + y² + ax + by - 14 = 0 has the
same centre as the first circle.
(iv) Write down the value of a and of b.
(v) Show that the second circle lies
inside the first circle.

Respuesta :

The equation of the first circle is (x + 1)^2 + (y - 1)^2 = r^2 and the equation of the second circle is (x + 1)² + (y - 1)² = 16

The equation of the perpendicular bisector

The points are given as:

P(3, 0) and Q(0, 5)

The midpoint of PQ is

Midpoint = 0.5(3 + 0, 0 + 5)

Midpoint = (1.5, 2.5)

Calculate the slope of PQ

m = (y2 - y1)/(x2 - x1)

m = (5 - 0)/(0 - 3)

m = -5/3

A line perpendicular to PQ would have a slope (n) of

n = -1/m

This gives

n = -1/(-5/3)

n = 0.6

The equation is then calculated as:

y = n(x - x1) + y1

Where

(x1, y1) = (1.5, 2.5)

So, we have:

y = 0.6(x - 1.5) + 2.5

y = 0.6x - 0.9 + 2.5

Evaluate the sum

y = 0.6x + 1.6

Hence, the equation of the perpendicular bisector of PQ is y = 0.6x + 1.6

The center of the circle

We have:

y = x + 2

Substitute y = x + 2 in y = 0.6x + 1.6

x + 2 = 0.6x + 1.6

Evaluate the like terms

0.4x = -0.4

Divide

x = -1

Substitute x = -1 in y = x + 2

y = -1 + 2

y = 1

Hence, the center of the circle is (-1, 1)

The circle equation

We have:

Center, (a, b) = (-1, 1)

Point, (x, y) = (0, 5) and (3, 0)

A circle equation is represented as:

(x - a)^2 + (y - b)^2 = r^2

Where r represents the radius.

Substitute (a, b) = (-1, 1) in (x - a)^2 + (y - b)^2 = r^2

(x + 1)^2 + (y - 1)^2 = r^2

Substitute (x, y) = (0, 5) in (x + 1)^2 + (y - 1)^2 = r^2

(0 + 1)^2 + (5 - 1)^2 = r^2

This gives

r^2 = 17

Substitute r^2 = 17 in (x + 1)^2 + (y - 1)^2 = r^2

(x + 1)^2 + (y - 1)^2 = r^2

Hence, the circle equation is (x + 1)^2 + (y - 1)^2 = r^2

The value of a and b

The equation of the second circle is

2x² + y² + ax + by - 14 = 0

Rewrite as:

2x² + ax + y² + by = 14

For x and y, we use the following assumptions

2x² + ax = 0    and  y² + by = 0

Divide through by 2

x² + 0.5ax = 0    and  y² + by = 0

Take the coefficients of x and y

k = 0.5a                   k = b

Divide by 2

k/2 = 0.25a           k/2 = 0.5b

Square both sides

(k/2)² = 0.0625a²           (k/2)² = 0.25b²

Add the above to both sides of the equations

x² + 0.5ax +0.0625a² = 0.0625a²    and  y² + by + 0.25b² = 0.25b²

Express as perfect squares

(x + 0.25a)² = 0.0625a² and (y + 0.5b)² = 0.25b²

Add both equations

(x + 0.25a)² + (y + 0.5b)² = 0.0625a² + 0.25b²

So, we have:

2x² + ax + y² + by = 14 becomes

(x + 0.25a)² + (y + 0.5b)² = 0.0625a² + 0.25b²+ 14

Comparing the above equation and (x + 1)^2 + (y - 1)^2 = r^2, we have:

0.25a = 1 and 0.5b = -1

Solve for a

a = 4 and b = -2

This means that the value of a is 4 and b is -2

Show that the second circle is in the first

We have:

a = 4 and b = -2

Substitute these values in (x + 0.25a)² + (y + 0.5b)² = 0.0625a² + 0.25b²+ 14

This gives

(x + 0.25*4)² + (y - 0.5*2)² = 0.0625*4² + 0.25*(-2)²+ 14

(x + 1)² + (y - 1)² = 16

The equation of the first circle is

(x + 1)² + (y - 1)² = 17

The radii of the first and the second circles are

R = √17

r = √16

√17 is greater than √16

Since they have the same center, and the radius of the first circle exceeds the radius of the second circle, then the second circle lies inside the first circle.

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