Circle equations

**delicate_tears** · April 24th 2008, 07:02 PM

1. Find the equation of the diameter of the circle x^2 + y^2 - 4x + 6y = 14 which passes through the origin.

2. Find the equation of the diameter of the circle x^2 + y^2 -3x + 2y = 26 which cuts the x-axis at an angle of 45 degrees.

**earboth** · April 24th 2008, 10:11 PM

Originally Posted by delicate_tears

1. Find the equation of the diameter of the circle x^2 + y^2 - 4x + 6y = 14 which passes through the origin.

2. Find the equation of the diameter of the circle x^2 + y^2 -3x + 2y = 26 which cuts the x-axis at an angle of 45 degrees.

to #1.:

Calculate the coordinates of the centre of the circle by completing the squares:

$x^2-4x {\color{red}+4} +y^2 + 6y {\color{red} + 9} = 14 {\color{red} +4 +9}~\implies~ (x-2)^2+ (y+3)^2=27$

The line in question has to pass through (2, -3) and the origin.

You should get: $y = -\frac32 x$

to #2:

Calculate the coordinates of the centre of the circle by completing the squares: $\left(x-\frac32\right)^2+(y+1)^2=\frac{117}4$

There are two cases possible if the x-axis and a straight line include an angle of 45°: The slope of the line can be m = 1 or m = -1

Use the point-slope-formula to calculate the equation of the line:

a) m = 1:

$y-1=\left(x-\frac32\right)~\implies~ \boxed{y=x-\frac12}$

b) m = -1

$y-1=-\left(x-\frac32\right)~\implies~ \boxed{y=-x+\frac52}$

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