- Topic
- Limits
- Difficulty
- Challenging
- Suggested time
- 5 min
- Marks
- 3
Limits practice question 21
Evaluate the limit.
A difference of two square roots sits on top. What do the top and the bottom do at , and how do you clear roots from a difference?
Substitution gives . Multiply the top and the bottom by the conjugate .
The new numerator is . Simplify it and look for a factor of .
A fraction or a decimal.
Theory UBC Math 100 §2.1.1CLP-1 §1.4Key results
Worked solution
Strategy
A difference of two roots calls for the conjugate, which squares each root away. The numerator then shares the factor x − 2 with the denominator.
Working
Multiply by the conjugate over itself, which is 1.
The bracket around x + 14 makes the 14 negative.
Cancel x − 2, then substitute.
Answer
The answer is one half.
Check
At x = 2.01 the quotient is (√16.05 − √16.01)/0.01 ≈ 0.4995, close to 1/2.
Where this goes wrong
Writing the conjugate sum at x = 2 as 4 instead of 4 + 4 = 8. Both roots equal 4 there, and both stay in the denominator.
Leans on
Rationalising with a conjugate, from Functions, graphs and inverses
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