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You might wish to recall the form of the pdf for an exponential
distribution with parameter $\lambda$ is

$$

f(x) = \lambda e^{-\lambda x}

$$

Also, this is a probability density function. What does this say about the area under the graph?

$$

f(x) = \lambda e^{-\lambda x}

$$

Also, this is a probability density function. What does this say about the area under the graph?