Why do this problem?
This is a non standard example of a quadratic inequality, the
solution of which will involve algebraic manipulation. It can
be used to help learners to practise the skills of estimation
and approximation prior to engaging in an algebraic
solution.
Possible
approach
There are various ways in which this problem can be tackled.
It can obviously be set 'straight' to learners, or used in a
slightly wider context, as described here.
To begin with, can learners assess a rough range of values
that is likely to emerge upon solving the inequality? For
example, would very large or very small values of $x$ satisfy
the inequality? As a class, who can spot the largest or
smallest values which will satisfy the inequality. This might
naturally lead to a numerical investigation, although in this
case it turns out that there is an exact algebraic solution
to the problem.
Once the problem has some sense of a numerical estimation,
encourage the class to move on to an algebraic solution.
Learners will soon discover that they need to deal with the
awkward $\sqrt{x}$ terms. One way is to square both sides of
the inequality; another is to make the substitution $\sqrt{x}
= p$. The inequality can then be rearranged into a quadratic
inequality.
It is important for learners not just to blindly apply rules
of algebraic manipulation to the resulting inequalities. For
each manipulation (squaring, rearranging etc.) learners
should explain clearly why the operation preserves the
inequality sign.
The next step will be to try to solve the quadratic
inequality. Learners might use an algebraic approach or a
graphical approach, but in each case will be required to use
the quadratic equation formula to find the solutions. It may
be helpful for the class to work together to solve the
inequality. A fruitful class discussion, where the learners
are able to volunteer suggestions for the steps in the
working, is more likely to ensue if they have at least
started solving the problem for themselves and have worked
through the earlier steps.
Finally, once a solution is found it is good practice to check
that the boundaries of the inequality work and also to compare
these to the original estimates.
Key questions
Can you estimate an approximate range of values for which the
inequality is satisfied?
What things might we try to remove the awkward $\sqrt{x}$
term?
How do we find the factors of a quadratic expression?
If you know that the product of two factors is negative what
can you say about the factors?
Possible extension
Can you make up a similar inequality which has solutions $3<
x < 5$?. How about $a < x < b$?
Possible support
Explicity suggest that learners substitute $\sqrt{x}= p$.