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**Checking solutions -- textbook wrong about roots?**

If I have the equation sqrt(3x + 1) = x - 3 and I need to solve for x, by squaring both sides then solving the resulting quadratic, I get the solutions x = 1, 8

However, since I squared the equation, I need to check if the solutions are valid. My calculus textbook says that x = 1 is not a valid solution as the LHS (sqrt(3(1) + 1)) = 2 and the RHS (1 - 3) = -2

However, the LHS also equals -2 as the square root of 4 is +/- 2.

So, is my textboox wrong? Or have I got the wrong idea somehow?