A circle has a center that falls on the line y = 2/3x +7 and passes through (5 ,2 ) and (3 ,2 ). What is the equation of the circle?

1 Answer
Aug 24, 2016

Write a systems of equations, letting (x, y) be the centre. The logic is the following: since the two points given lie on the circumference of the circle, the distance to the centre is equal with both points, so let equation 1 be sqrt((x - 5)^2 + (y - 2)^2) = sqrt((x - 3)^2 + (y - 2)^2)

The centre lies on the line y = 2/3x + 7, so it makes sense that our second equation is y = 2/3x + 7.

Let's now solve using substitution.

sqrt((x - 5)^2 + (2/3x + 7 - 2)^2 ) = sqrt((x - 3)^2 + (2/3x + 7 - 2)^2)

x^2 - 10x + 25 + 4/9x^2 + 20/3x + 25 = x^2 - 6x + 9 + 4/9x^2 + 20/3x + 25

-4x = -16

x = 4

y = 2/3(4) + 7 = 8/3 + 7 = 29/3

Hence, the centre has coordinates of (4, 29/3).

Now, we need to determine the measure of the radius, again using the distance formula.

r = sqrt((5 - 4)^2 + (29/3 - 2)^2)

r = sqrt(1 + 529/9)

r = sqrt(538/9)

The most commonly used form of the equation of the circle is (x - a)^2 + (y - b)^2 = r^2, where (a, b) is the centre and r the radius.

Thus, the equation of the circle is (x - 4)^2 + (y - 29/3)^2 = 538/9

Hopefully this helps!