Criteria for arrangement or combination of identification
The combinations exercises can be solved by arrangement or combination, but to identify which of the two groups exercise is referring? This requires us to put in place some criteria that will help in this identification.
These criteria are applied as follows: In a combinations problem will find several groups, mount at least one of them and change the order of the elements of this group.
If after the change has formed a different group, this issue will be arrangement.
If after the change has formed the same cluster, this problem will be combined, or even if the elements in different order to continue to identify the same group.
See how the application of this criterion works:
Consider nine different points of a circle, as pictured.
How many lines are determined by these nine points?
To find out if the exercise is arrangement or combination we need to make at least one of the groups (straight).
A line is made up, to say the least, 2 points, as points are not col linear can join any point, so we can say that (A, B) is a grouping, if we change the order of its elements (B, A) straight (grouping) remains the same, so this exercise will be solved by combining.
Thus, we apply the formula combining, where n = 9 and p = 2.
C9,2 = 36
They will be formed with the 9 points of the circumferences 36 straight