## Plotting points on a graph

Graphs usually have points or markers on them. The location of this point is given by an ordered pair.

Graphs, sometimes known as gryphs, usually have points or markers on them. The location of this point is given by an ordered pair. For instance ordered pairs are used to locate points on a street map. For street maps, however, it is common to use a letter and a number, say B17 or D5. An ordered pair on a graph is a pair of numbers in a certain order written in brackets and separated by a comma. For example, six comma four is an ordered pair. When an ordered pair is used to locate a point on a grid the two numbers are called the co-ordinates of the point. The point at the centre of the graph is called the origin. The co-ordinates of this point are zero comma zero, this is the point at which the horizontal X axis intersects the vertical Y axis.

Let’s look at point A, to find the X co-ordinate we start at the origin and move four units horizontally to the right. To find the Y co-ordinate we move vertically up two units, the co-ordinates of A are therefore four comma two. Note that the order of the numbers is very important, for instance four comma two is a very different point on a graph to the ordered pair two comma four.

Now let’s look at point B. To find the X co-ordinate we move from the origin two units to the left which means it’s a negative X value, ie, negative two. To find the Y co-ordinate we move above the origin four units which means it’s a positive Y value, ie, four or positive four. Hence the co-ordinates of B are negative two comma four.

Now let’s look at point C. To find the X co-ordinate we move from the origin six units to the left, which means it’s a negative X value, ie, negative six. To find the Y co-ordinate we move below the origin four units, which means it’s a negative Y value, ie, negative four. Hence the co-ordinates of C are negative six comma negative four.

Finally let’s look at point D. To find the X co-ordinate we move from the origin three units to the right which means it’s a positive X value, ie, three. Notice that this point is midway between two and four on the X axis. To find the Y co-ordinate we move below the origin three units, which means it’s a negative Y value, ie, negative three. Again we have taken the point midway between two and four. Hence the co-ordinates of D are three comma negative three.

That finishes the lessons on graphs. Now open the graphs worksheet and work through the exercises. Thank you.

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