Practice Final Exam

r² + 4 ≠ zero because any number substituted for r will be squared and hence always positive.
If -2 is substituted for r the answer would be (-2)² + 4 = 4 + 4 = 8
Another way of looking at the problem is if you solve r² + 4 = 0  r² = -4 then the square of a number must equal a negative number. Which is impossible in our mathematical system.
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