# Circles,points

• Dec 11th 2007, 07:06 AM
Chez_
Circles,points
A(9,8) B(5,0) c(3,1) are three points, how can i show that AB and BC are perpendicular , also from this how can i find the equation of the circle with AC as diameter? and show that B lies on this circle? Thank you
• Dec 11th 2007, 07:15 AM
topsquark
Quote:

Originally Posted by Chez_
A(9,8) B(5,0) c(3,1) are three points, how can i show that AB and BC are perpendicular , also from this how can i find the equation of the circle with AC as diameter? and show that B lies on this circle? Thank you

Lots of questions.

First, to show that AB and BC are perpendicular, calculate the slope of the line between A and B. Call this m1. Then calculate the slope between B and C. Call this m2. If the two lines are perpendicular then $\displaystyle m_1 \cdot m_2 = -1$.

For the second question. AC is a diameter, so to find the center of the circle you need the midpoint of AC. You can find this by the formulas:
$\displaystyle x_{mid} = \frac{x_A + x_C}{2}$
and
$\displaystyle y_{mid} = \frac{y_A + y_C}{2}$

The radius will be half the diameter, which is AC. So find the length of AC and divide it by 2. Call this r.

The equation of the circle of which AC is a diameter is
$\displaystyle (x - x_{mid} )^2 + (y - y_{mid} )^2 = r^2$

Finally, to show that B lies on this circle, all you need to do is plug the coordinates of B into the circle equation you just wrote down. If the LHS and RHS match, then B is on the circle.

-Dan