How do you solve and graph x^3<=4x^2+3x?

1 Answer
Jun 25, 2017

The solution is x in (-oo, 2-sqrt7] uu [0, 2+sqrt7]

Explanation:

We solve this inequality with a sign chart.

x^3<=4x^2+3x

x^3-4x^2-3x<=0

x(x^2-4x-3)<=0

We need the roots of the quadratic equation

x^2-4x-3=0

The discrimenant is

Delta=b^2-4ac=(-4)^2-4*(1)*(-3)=16+12=28

As, Delta>0, there are 2 real roots

x_2=(-b+sqrtDelta)/2=1/2(4+sqrt28)=2+sqrt7=4.646

x_1=(-b-sqrtDelta)/2=1/2(4+sqrt28)=2-sqrt7=-0.646

Let f(x)=x(x-x_1)(x-x_2)

We can build the sign chart

color(white)(aaaa)xcolor(white)(aaaa)-oocolor(white)(aaaa)x_1color(white)(aaaa)0color(white)(aaaaaa)x_2color(white)(aaaa)+oo

color(white)(aaaa)x_1color(white)(aaaaaaa)-color(white)(aaaa)+color(white)(aaaa)+color(white)(aaaa)+

color(white)(aaaa)xcolor(white)(aaaaaaaa)-color(white)(aaaa)-color(white)(aaaa)+color(white)(aaaa)+

color(white)(aaaa)x_2color(white)(aaaaaaa)-color(white)(aaaa)-color(white)(aaaa)-color(white)(aaaa)+

color(white)(aaaa)f(x)color(white)(aaaaaa)-color(white)(aaaa)+color(white)(aaaa)-color(white)(aaaa)+

Therefore,

f(x)<=0 when x in (-oo, 2-sqrt7] uu [0, 2+sqrt7]
graph{x^3-4x^2-3x [-12.34, 12.97, -9.01, 3.65]}