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There are two types of relationships between two items: functional relationships and correlations. In this section, we describe functional relationships.
Functional relationship
Assume that the infusion volume is 60 mL for 1 h. The infusion volume will then be 120 mL for 2 h, 180 mL for 3 h, and 240 mL for 4 h.
If we plot the time required for infusion on the x-axis (horizontal axis) and the volume injected on the y-axis (vertical axis), we can draw a graph as shown in Figure 1.
[Figure 1] Examples of functional relationships
The time required for infusion (x) and the infusion volume (y) is expressed as y=60x. For example, the infusion amount after 5 h can be calculated by substituting 5 for x.
The actual calculation is as follows:
y=60×5=300 mL
In this way, when the value of y is determined according to the value of x, we say that there is a functional relationship between x and y. When the relationship y=ax+b holds between x and y, we say that “y is a linear function of x.”
In addition, if we graph the relationship between the linear functions, it appears as a straight line, as shown in Figure 2.
[Figure 2] Graph of linear function
A and b are constants; a is the slope of the line, and b is the value of the coordinate where the line intersects the y-axis.
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