# Coordinate Line

Let′s draw the line `l` and will note on it the point `O`, which we take as the origin. Thereafter we must to choose the direction and the unit segment.

In this case we say, that we have **coordinate line**. Every naturall number or fraction corresponds to one point of line `l`. Suppose, for example, it is 3. Let′s measure from the point `O` the unit segment three times in a given direction and we will get the point `A` - and it corresponds to the number 3. Analogically, if we have the number 4.2 we will measure it from point `O` the unit segment four times in a given direction and then we measure 0.2 more of the parts of this segment and we will obtain the point `B` - it corresponds to 4.2.

If the point `M` of line `l` corresponds to number `r` then this number is called** coordinate point** and we write `M (r)`. So, for the points `J, A, B` we can show their coordinates: `J (1), A (3), B (4.2)`. The coordinate of point `O` is zero.

Let′s measure from the point the unit segment three times in a given direction, that is opposite of given. We will obtain the point `A`′, that is symmetric point `Ð`relative to the origin. The coordinate of point `A` is 3, and the coordinate of point `A`′ we write -3 and read "minus 3". Analogically, the coordinate of point `B`′ that is symmetric of the point `B` is number -4.2. The numbers 3 and -3, 4.2 and -4.2 are called **opposite**. The numbers which corresponds to the points, that are on the line in a given direction are called** possitive**, so 1, 3, 4.2 are possitive numbers. The numbers which corresponds to the points, that are on the line in a opposite to the given direction are called **negative**, so -3, -4.2 are negative numbers. Zero is considered neither positive nor negatine number.

The given direction on the coordinate line is called **positive** (it is usually directed to the right) and the direction opposite to the given is **negative** (it is directed to the left).