why is the area under unit impulse is always equal to 1?

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I have seen other questions related to my question. I had this question when I was solving the laplace transform of unit impulse function in the book : Principles and practice of automatic process control. Now I understand how we used the formula so I'm okay with the calculation. But graphically, how is the area of a straight line extending to infinity equal to one ?! Thank you for your help

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