Is the Inequality Valid with Positive x?

[s3a]

Homework Statement

Hello, everyone. :)

I'm having trouble with problem 2.26(b) from the attached PDF file.

Homework Equations

d^2 y/dt^2 = 6Hu^2/L^2 - 12Hxu^2/L^3
d^2 y/dt^2 <= A
6Hu^2/L^2 - 12Hxu^2/L^3 <= A
6Hu^2/L^2 <= A

The Attempt at a Solution

I don't understand why the fact that x > 0 means that the term -12Hxu^2/L^3 in d^2 y/dt^2 = 6Hu^2/L^2 - 12Hxu^2/L^3 can be ignored.

To me, it seems that removing it makes the inequality go from some_quantity <= A to some_larger_quantity <= A. How does one justify that?

Any input would be greatly appreciated!

 

[Mark44]

To me, it seems that removing it makes the inequality go from some_quantity <= A to some_larger_quantity <= A. How does one justify that?

It doesn't make any sense to me, either.
If you have x - b <= A, with b being positive, it doesn't necessarily follow that x <= A.
Simple example: 12 - 3 <= 10, but 12 > 10.