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Zhang Li solves the equation below by first squaring both sides of the equation.

-4=sqrt(5z+7)
What extraneous solution does Zhang Li obtain?

z=

Zhang Li solves the equation below by first squaring both sides of the equation.\newline4=5z+7 -4=\sqrt{5 z+7} \newlineWhat extraneous solution does Zhang Li obtain?\newlinez= z=

Full solution

Q. Zhang Li solves the equation below by first squaring both sides of the equation.\newline4=5z+7 -4=\sqrt{5 z+7} \newlineWhat extraneous solution does Zhang Li obtain?\newlinez= z=
  1. Square both sides: Square both sides of the equation to eliminate the square root.\newline4=5z+7-4 = \sqrt{5z + 7}\newline(4)2=(5z+7)2(-4)^2 = (\sqrt{5z + 7})^2\newline16=5z+716 = 5z + 7
  2. Subtract 77 to isolate zz: Subtract 77 from both sides to isolate the term with zz.167=5z+7716 - 7 = 5z + 7 - 79=5z9 = 5z
  3. Divide both sides by 55: Divide both sides by 55 to solve for zz.95=5z5\frac{9}{5} = \frac{5z}{5}z=95z = \frac{9}{5}
  4. Check the solution: Check the solution by substituting it back into the original equation.\newline4=5(95)+7-4 = \sqrt{5(\frac{9}{5}) + 7}\newline4=9+7-4 = \sqrt{9 + 7}\newline4=16-4 = \sqrt{16}\newline4=4-4 = 4 or 4=4-4 = -4\newlineHere we see that the original equation has a negative number on one side and a square root on the other side, which cannot be negative. Therefore, the solution z=95z = \frac{9}{5} is extraneous because it leads to a false statement when substituted back into the original equation.

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