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## Functions as Black Boxes

- Many functions have input values or parameters that are transferred or passed into the function.
  - The `x` in `y = sqrt(x);`
  - Both the `x` and the `y` in `z = pow(x, y);`
  - Could be an expression as in `sqrt(b * b - 4 * a * c);`
- Many functions have output or return values.
  - The `y` in `y = sqrt(x);`
- Both parameters and return values are a particular type.
  - You cannot compute `sqrt("Harry");`
  - You cannot assign `string s = sqrt(5.5);`

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