Mathematics

Mathematics

Problem

Suppose dydx=3x2+4x3+4x\dfrac{dy}{dx}=\dfrac{3x^{2}+4}{x^{3}+4x} for x>0x>0 and y(1)=0y(1)=0. Which of the following gives y(x)y(x) for x>0x>0? a) y(x)=ln⁡ ⁣(x2+4)−ln⁡5y(x)=\ln\!\left(x^{2}+4\right)-\ln 5 b) y(x)=ln⁡ ⁣(x(x2+4)5)y(x)=\ln\!\left(\dfrac{x(x^{2}+4)}{5}\right) c) y(x)=ln⁡ ⁣(x2+4x)−ln⁡5y(x)=\ln\!\left(\dfrac{x^{2}+4}{x}\right)-\ln 5 d) y(x)=15 ln⁡ ⁣[x(x2+4)]y(x)=\dfrac{1}{5}\,\ln\!\left[x(x^{2}+4)\right]
Intuitive explanation

This is a separable differential equation where the key insight is recognizing that the numerator is almost the derivative of the denominator. Notice that if you differentiate x3+4xx^3 + 4x, you get 3x2+43x^2 + 4—exactly your numerator! This means you can integrate both sides by recognizing the natural logarithm pattern: ∫d(u)u=ln⁡∣u∣+C\int \frac{d(u)}{u} = \ln|u| + C. Once you find the general solution, use the initial condition y(1)=0y(1) = 0 to pin down the constant.

Intuitive explanation

This is a separable differential equation where the key insight is recognizing that the numerator is almost the derivative of the denominator. Notice that if you differentiate x3+4xx^3 + 4x, you get 3x2+43x^2 + 4—exactly your numerator! This means you can integrate both sides by recognizing the natural logarithm pattern: ∫d(u)u=ln⁡∣u∣+C\int \frac{d(u)}{u} = \ln|u| + C. Once you find the general solution, use the initial condition y(1)=0y(1) = 0 to pin down the constant.

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