# A-level Mathematics/CIE/Pure Mathematics 1/Coordinate Geometry

## The Equation of a Line

edit### Calculations involving lines

edit#### Distance between two points

editThe **distance** between two points is given by the formula where is the difference in *x-values* between the two points and is the difference in *y-values* between the two points.

This formula can also be seen as applying the Pythagorean theorem to the points, where the differences in x-values and y-values form two sides of a right-angled triangle.

#### Midpoint of two points

editThe **midpoint** is the point which is exactly halfway between two points. The coordinates of the midpoint are given by where and are the coordinates of the two points.

e.g. The midpoint between and is

You may notice that this expression states that the midpoint's x-coordinate is the average of the x-coordinates of the points, and its y-coordinate is the average of the points' y-coordinates. Essentially, this means that the midpoint is the average of the two points.

#### Gradients

editThe **gradient** of a line is determined by the ratio where is the change in y-value and is the change in x-value.

This can also be expressed as when finding the gradient of a line between two points.

e.g. The gradient of a line that passes through and has gradient

#### Intersecting lines

editWhen two lines **intersect**, the point of intersection is where the two lines cross. The point of intersection is thus on both lines, meaning that it can be found using simultaneous equations.

e.g. The lines and intersect. Find the point of intersection.

#### Parallel lines

edit**Parallel** lines always have the same gradient, and do not intersect.

e.g. The lines and are parallel.

Sometimes we'll need to find a line which is parallel to a given line and passes through a given point.

e.g. Find the equation of a line parallel to that passes through

#### Perpendicular lines

edit**Perpendicular** lines are at right angles to one another. The product of the gradients of two perpendicular lines is always -1.

e.g. The lines and are perpendicular.

Sometimes we'll need to find a perpendicular line that goes through a specific point.

e.g. Find the equation of the line perpendicular to that passes through the origin.

### Different forms of the equation of a line

editThere are three main ways to write an equation of a line:

### Finding the equation of a line from a point and the gradient

editThe equation of a line can be found using a point and the gradient using the second equation followed by rearranging the equation to the form .

e.g. A line with gradient goes through the point . Find its equation.

### Finding the equation of a line from two points

editWhen given two points, we can find the gradient using . Using this gradient, the same method can be used as for a point and the gradient.

e.g. A line travels through the points and . Find the equation of the line.

## The Equation of a Circle

editA circle consists of all points that are a given distance from its centre. The distance between two points can be defined using the Pythagorean theorem . Thus, the equation of a circle centred at the origin is given by where is the radius of the circle.

e.g. A circle centred at the origin with radius would have the equation

If the circle is not centred at the origin, we can translate this equation to a different point. Thus, the equation becomes where are the coordinates of the centre.

e.g. A circle centred at with radius would have the equation

## Interactions between Lines & Circles

editWhen given a problem where a line and a circle intersect, it is useful to use a substitution method of solving simultaneous equations.

e.g. The line intersects the circle . Find the coordinates of these intersection points

## Interactions between Lines & Quadratics

editWhen a quadratic and a line intersect, we can again use substitution to find the points of intersection.

e.g. The line intersects the quadratic . Find the points of intersection.

In some cases, we need to find a constant that ensures there is only one point of intersection. In said cases, we should use the discriminant.

e.g. Find the value such that the line is tangent to