How do you solve the inequality 6x^2-5x>6?

1 Answer
Feb 4, 2015

I would start by solving it as a normal equation (2nd degree). So you write:
6x^2-5x-6>0 (I took the 6 to the left);
Solving your equation you should get two values x_1=3/2 and x_2=-2/3.
Now the tricky bit:
your inequality asks you for values of x that make your equation have a value bigger than zero.
They cannot be x_1 and x_2 because at these point your equation IS equal to zero.
Graphically your function gives the following parabola:

graph{6x^2-5x-6 [-11.96, 13.02, -7.14, 5.34]}

So, basically I have to choose values that are outside the boundaries formed by the two values x_1 and x_2 to get a value bigger than zero (in the graph the two bits that are ABOVE the x axis).!!!
Consider x_1=3/2=1.5 ok I cannot choose it but what about 2 (which is bigger)?
If I put x=2 in the equation I get: 6*4-5*2-6=8>0 YES!
Consider now x_2=-2/3=-0.67 again I cannot choose it but what about -1?
If I put x=-1 in the equation I get: 6*(-1)^2-5*(-1)-6=5>0 YES!
So, OUTSIDE the interval bound by x_1 and x_2 you can choose x.
You express this by writing your solution as:
-2/3>x>3/2
Or graphically
enter image source here

Hope it helps