Answer:
The square roots of 400 are 20 and -20.
Explanation:
You can try the offered answers to see if they square to give 400:
(-40)² = 1600 . . . . -40 is not a square root of 400
(-20)² = 400 . . . . -20 is a square root of 400
20² = 400 . . . . . 20 is a square root of 400
40² = 1600 . . . . 40 is not a square root of 400
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The second degree equation ...
x² = 400
will have exactly two solutions, as required by the fundamental theorem of algebra. Those solutions are both given above. There are no other square roots of 400, real or complex.