Posted by Rinchan on Sunday, October 20, 2013 at 8:57am.
Solve the inequality 12x^3 + 8x^2 <= 3x + 2. Justify your answer.

advanced functions  Reiny, Sunday, October 20, 2013 at 9:28am
12x^3 + 8x^2 ≤ 3x + 2
12x3 + 8x^2  3x  2 ≤ 0
after some trial and errors where I tried
x = ±1, ±2, etc, I found
x = ±1/2 and x = 2/3 give me zero
12x3 + 8x^2  3x  2 ≤ 0
(2x1)(2x+1)(3x+2) ≤ 0
knowing the behaviour of
y = 12x3 + 8x^2  3x  2
and having xintercepts of 2/3, 1/2 and +1/2
the solution would be
x ≤ 2/3 OR 1/2 ≤ x ≤ 1/2
confirmation:
http://www.wolframalpha.com/input/?i=plot+12x3+%2B+8x%5E2++3x++2+≤+0