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weird maths sum!!

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The questions is simply: Find the range of values of x for which x(2-x)<-15

This is what i did:

x(2-x)+15<0
2x - x^2 + 15 < 0
-x^2 + 2x + 15 < 0
(x+3)(-x+15) < 0
-3<x<5

but when i check by subbing the number, say, 4 into x, the answer is always greater than zero. And the correct answer should be x<3 or x>5, which also seems wrong, because when i sub a number, say 2, into the inequality, the answer is still greater than 0.

what is wrong??? I've checked the things i did so many times and i don't see anything wrong????

  • weird maths sum!! - ,

    Look at this:
    (x+3)<0
    x<-3
    Not greater than.
    -x+5<0
    x>5

    so x<-3 and x>5

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