For every four hours  patient receives a  mg injection of a drug.

Find .

Observe the graph.

As  approaches to  from the left side,  approaches to .

So .

is the amount of the drug just before the fourth injection is given.

Find .

Observe the graph.

As approaches to from the right side, approaches to .

So .

is the amount of the drug after the fourth injection is received by the patient.

is the amount of the drug just before the fourth injection is given.

is the amount of the drug after the fourth injection is received by the patient.