Posted by Bailey on .
The questions is simply: Find the range of values of x for which x(2x)<15
This is what i did:
x(2x)+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!! 
bobpursley,
Look at this:
(x+3)<0
x<3
Not greater than.
x+5<0
x>5
so x<3 and x>5