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Analytic Geometry: (lesson 1 of 4)

Basic Concepts

Distance

The distance between two points in the plane is the length of the line segment joining the two points. If the points have Cartesian coordinates (x1,x2)(x_1, x_2) and (y1,y2)(y_1, y_2), then

d=(x1x2)2+(y1y2)2 d = \sqrt{(x_1 - x_2)^2 + (y_1 - y_2)^2}

If the points have polar coordinates (r1,θ1)(r_1, \theta_1) and (r2,θ2)(r_2, \theta_2), then

d=r12+r22+2r1r2cos(θ1θ2) d = \sqrt{{r_1}^2 + {r_2}^2 + 2 r_1 r_2 \cos (\theta_1 - \theta_2)}

Midpoint of two points

The midpoint of two points (x1,x2)(x_1, x_2) and (y1,y2)(y_1, y_2) is

(x1+x22,y1y22) \left( \frac{x_1 + x_2}{2}, \frac{y_1 - y_2}{2} \right)