Do you remember having to memorize formulas for area and volume of various 2d and 3d shapes? Calculus will let you figure out those formulas for yourself, when you run across a shape nobody gave you the formula for, or just can’t be bothered to look it up.
From an engineering perspective, you might need to figure out the volume of a tank or displacement of a ship from its shape, or you might have the measured power output of an engine and need to figure out how much energy it produced over the whole test run, so you can calculate efficiency.
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