4. The 3-Part Definition of Continuity at a point.
A function f is continuous at x = a then we must have the following three
conditions.
1. ƒ(a) is defined; in other words, a is in the domain of ƒ.
2. The limit must exist.
3. The two numbers in 1. and 2., ƒ(a) and L, must be equal
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