When is the inequality x^4 < 8x^2 + 9 true?
If we consider this like a normal quadratic problem, this becomes easy
x4 < 8x2 + 9
x4 - 8x2 - 9 < 0
(x2-9)(x2+1) < 0
This means there are roots of this expression at x2 = 9 and x2 = -1
Since for all reals, x2 > 0, we know the two roots of this expression are x=+-3
Now, since x4 - 8x2 - 9 is a quartic (ie, it has an x4 expression), we know that given any sufficiently positive or negative x, the quartic will be positive (ie, if x is 10000, or -10000)
Therefore, we know for this to be true, -3<x<3 (since we have found the solutions, we simply need to work out which regions satisfy the criteria)
JP
