Many students make this error because they do not understand what the radical sign indicates. Often this goes back to their understanding (or misconception) of a perfect square. For example, students learn that 100 is a perfect square because it is the product of 10 and itself. Then, we write the equation: √100 = 10. After showing them a few of these, ask students to list the perfect squares. Many of them will give √100 instead of 100. Or, how many times have I seen students “keep on going” as long as the radicand is a perfect square? ( √16 = √4 = √2 )
Maybe it’s related to vocabulary as well. Have you seen this?

Teacher: Find the square of 9

Student: 3

Teacher: Show me how you get that.

Student: √9 = 3

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