Thread: Polynomial inequalities

I have a question I worked out the problems on the bottom but it says my answers are wrong. Can you please tell me what I did wrong?

The first problem is the first attached file for it I got the answer [2,4] by doing the sign chart

The second problem is the second file attached for it I got the answer [-4,0]U[8,13]

The third problem is the third file attached and for it I got (-infinity,-6)U(6,infinity)

The fourth problem is the fourth file attached and for that I got (-infinity,-13]U[4,infinity)

All those answers are wrong, so can you tell me what I did wrong?

Originally Posted by Girlaaaaaaaa

I have a question I worked out the problems on the bottom but it says my answers are wrong. Can you please tell me what I did wrong?

The first problem is the first attached file for it I got the answer [2,4] by doing the sign chart

The second problem is the second file attached for it I got the answer [-4,0]U[8,13]

The third problem is the third file attached and for it I got (-infinity,-6)U(6,infinity)

The fourth problem is the fourth file attached and for that I got (-infinity,-13]U[4,infinity)

All those answers are wrong, so can you tell me what I did wrong?

Most of your answers are wrong because you include the case that the denominator equals zero. For instance:

The solution of the 1rst problem is (2, 4). With your solution you actually divide by zero

I've got a total different solution with the 3rd problem: or as an interval:

Originally Posted by earboth

Most of your answers are wrong because you include the case that the denominator equals zero. For instance:

The solution of the 1rst problem is (2, 4). With your solution you actually divide by zero

I've got a total different solution with the 3rd problem: or as an interval:

I just tried your answer for the third problem and it showed that it is incorrect also. Btw how did you get that answer?

Originally Posted by Girlaaaaaaaa

I just tried your answer for the third problem and it showed that it is incorrect also. Btw how did you get that answer?

You are right - but I am right too :

. You can multiply the inequaltity by (x-6) if you pay attention to the sign of the denominator:

that means

that means there isn't any real x which satisfies this case.

Hello,

For the first one, you can say that the inverse function decreases. This means that for every x & y, if 1/x < 1/y, 1/(1/x)=x > 1/(1/y)=y

So it's the same as solving x-4 > x-2, which is always false. So there is no solution...

For the third one, it's the same method. If i'm not mistaking, the answer is x<25/4, excluding x=6 because it's a zero of the denominator.

Here a comment about the usual way this taught in North America.
Solving students are encouraged to ‘compare to zero’.
.
It is easy to check to see what values that make the statement true.