Keep the denominator sign in view
Solve algebraically. marks Hence solve . marks
[2009/JJC/Prelim/I/6b]
Solution
Rational inequalities
Solve the inequality algebraically.
Hint
Do not cross-multiply: can be positive or negative.
Move everything to the left and combine into one fraction.
Factorise and use a sign chart; remember .
That is only valid when . If the inequality reverses, so you would need two separate cases.
and cannot both hold, giving the empty set. The solution is the union of the two intervals.
The original fraction is undefined there.
Answer · (a)
or .
Move everything to one side
Comparing with 0 lets a sign chart handle both possible denominator signs without separate cases.
Single fraction
, so the numerator is .
Make the leading coefficient positive
Multiplying both sides by reverses the inequality. Alternatively, multiply the resulting fraction inequality by for : this gives without changing its direction.
Sign chart
The critical values are (excluded), and . The expression is negative to the left of and between and .
Explore the idea
Substitution into a known result
Hence solve .
Hint
Use .
Compare with part (a): plays the role of .
Translate back to using exponentials. Keep .
only exists for positive , so the interval becomes .
, and .
Answer · (b)
or .
Match the form
for . This is exactly part (a) with replaced by .
Use the earlier answer
Substitute for in or .
Undo the logarithm
and, since requires , we get . And .