Use complete sentences to describe the relationship between sets a and b if a b.

a = {8}
b = {7, 8, 9}

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Set A is composed of the single member "8"

Set B has the three members "7", "8", and "9"

Notice how "8" is in both sets. We can say that set A is contained set B, or that set A is a subset of set B, because of this fact. Consider a Venn Diagram. Draw a circle representing set B. Fill in the circle with "7", "8", and "9". Then draw a smaller circle just around the "8" to represent this as set A.

Visually you can see that A is inside B completely. There are no portions of A that are outside of B. If we threw a dart and landed in A, then we're also inside B as well. This is the same as saying "we picked out '8' from a hat, so it came from set A or set B". 

So to summarize, A is a subset of B. With math notation, we would write this as . The "C" looking symbol (it's not really a letter C) means "subset".