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MathsMediumClass 12

Rate of Change

Applications of Derivatives

15

JEE Qs

8%

Hard

75

min

Master the art of translating word problems into mathematical relationships and applying the chain rule correctly for time-based rates.

🧮 Key Formulas

Rate of change of y with respect to x: dy/dx
Rate of change of y with respect to time t: dy/dt = (dy/dx) * (dx/dt) (Chain Rule)

✅ Key Points for JEE

  • 1Correctly identify the variables involved and the variable with respect to which the rate is to be found.
  • 2Formulate a relationship between the quantities whose rates are given or required, often using geometric formulas or problem-specific constraints.
  • 3Always differentiate the derived relationship with respect to the independent variable (often time 't') before substituting any constant values.
  • 4Pay attention to the sign of the rate: positive indicates increase, negative indicates decrease.
  • 5Units are crucial; ensure consistency and correct interpretation of derived rate units.

⚠️ Common Mistakes

  • Substituting numerical values for variables *before* differentiating, leading to an incorrect zero derivative (e.g., substituting r=5 into V=(4/3)πr³ and then differentiating, instead of differentiating V=(4/3)πr³ with respect to time first).
  • Incorrectly applying the chain rule, especially when differentiating implicit relationships with respect to time.
  • Errors in setting up the initial functional relationship between the variables, often due to misinterpreting the problem statement or using incorrect geometric formulas.

📝 Practice Questions

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NCERT Chapters

  • Class 12 Mathematics Ch 6: Applications of Derivatives