- Difficulty
- Standard
- Suggested time
- 4 min
- Marks
- 3
Implicit differentiation and inverse trigonometric functions practice question 18
Find in terms of and for the curve below.
Differentiate both sides with respect to . The term is a product of and , and needs the chain rule.
The product rule gives . Then move every term containing to one side, factor it out, and divide.
A formula in x and y. Use ^ for powers and brackets for every input: sin(2x), e^(x^2), log(x+1). Give exact values: 1/6, not 0.167.
Theory UBC Math 100 §4.5CLP-1 §2.11Key results
Worked solution
Strategy
Differentiate each side with respect to x, using the product rule on xy² and the chain rule on y² and eʸ, then solve the resulting linear equation for dy/dx.
Working
The product rule, with the chain rule on y²; the chain rule on eʸ.
The right side, 2x, differentiates to 2.
Keep the dy/dx terms on the left, move y² to the right, and factor.
Divide by the bracket.
Answer
The answer is dy/dx equals (2 minus y squared) over (2xy plus e to the y).
Check
At (½, 0) the term xy² is negligible, so the curve hugs y = log(2x), whose slope there is 2, and the formula gives 2/(0 + 1) = 2.
Where this goes wrong
Differentiating eʸ as eʸ with no dy/dx. eʸ depends on x through y, so its derivative is eʸ dy/dx, and that term belongs with the other dy/dx term.
Leans on
The product rule, from Differentiation rules
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