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How to find the domain of a function?

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2 Answers

The domain of √(x+5) is x ≥ -5, and the domain of √(x-1) is x ≥ 1.

The domain of f(x)=3√(x+5)+7√(x-1) is the conjunction of x ≥ -5 and x ≥ 1.

Answer: x ≥ 1.

 

Robert gave a good answer for your specific question. In general when asked to find the domain of a function, we are interested in values of x where we get a real valued answer. That means the inside of a radical expression must be non negative. So they can be 0 or 4 but not -3 or -14 for example inside of the radical, since the solution to negative radicals are complex and this outside of our domain.

Therefore the expression sqrt(x+5), the x+5 must be positive or zero, so the least number x can be is -5. When we consider both expressions, both must follow this rule which is why we get that x must be greater than or equal to -1.