The integrating factor of equation sec^2 y dy/dx + x tan y = x^3 is
- A. [e]^(x^2/2)
- B. [e]^(-x^2/2)
- C. [e]^(x/2)
- D. [e]^(-x/2)
Answer: A. [e]^(x^2/2)
Explanation: Substitute u = tan y, then du/dx + x·u = x³, which is a linear differential equation. The integrating factor is e^(∫ x dx) = e^(x²/2).