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Continuity & Differentiability · Question 123 of 125

What is the right-hand derivative of this function at x = 1?

Question 123

HardOriginal question

Let f(x)=x∣x∣+∣x−1∣f(x) = x\lvert x \rvert + \lvert x - 1 \rvert.

What is the right-hand derivative of this function at x=1x = 1?

Options for question 123

Show the answer and explanation
CorrectNot quiteAnswer D

Correct answer: D

Explanation

  1. For x>1x > 1, x∣x∣=x2x\lvert x \rvert = x^{2} and ∣x−1∣=x−1\lvert x - 1 \rvert = x - 1, so f(x)=x2+x−1f(x) = x^{2} + x - 1.

  2. Its slope at 1 is 2(1)+1=32(1) + 1 = 3.

Worth noting

1 is the left-hand derivative, the slope of x2+1−xx^{2} + 1 - x; the corner at 1 makes the two sides differ.

Concept
One-sided derivatives
Typical time
150 s
Your time
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The rule this testsDifferentiability
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Ten questions from Continuity & Differentiability, at your own pace.

Practise this chapter →

Ten questions from Continuity & Differentiability, at your own pace.