Question 150883
(2x-5)^2 = -16
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i is the sqrt of -1.
That means -1 is a perfect square, just like 4 and 9 and 16 are perfect squares.
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Historically it was introduced into mathematics when someone needed a 
solution to the following equation:
x^2+1 = 0
The answer had to be x^2 = -1
So x=i became the solution.
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Your Prolem:
(2x-5)^2 = -16
Take the square root of both sides to get:
2x-5 = +4i or 2x-5 = -4i
2x = 5 + 4i or 2x = 5 - 4i
x = (5+4i)/2 or x = (5-4i)/2
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I don't have your text so I can't talk about your other problem.
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Just remember that -1 is a perfect square; sqrt(-1) = i
Cheers,
Stan H.