Separation of Variables Error Analysis
A student attempts to solve the differential equation $$\displaystyle \frac{dy}{dx} = y \sin x$$ with the initial condition that $$y = 1$$ when $$x = 0$$. The steps of the student’s solution are shown below. In which of the following steps does an error first appear?
Step 1: $$\displaystyle \int \frac{1}{y} dy = \int \sin x dx$$
Step 2: $$\ln |y| = -\cos x + C$$
Step 3: Using the initial condition: $$\ln |1| = -\cos(0) + C$$, so $$C = 1$$
Step 4: $$\ln |y| = -\cos x + 1$$
Step 5: $$|y| = e^{-\cos x + 1}$$
Step 6: $$y = e \cdot e^{-\cos x}$$
Step 7: $$y = e^{1-\cos x}$$
Step 8: $$y = e^{\cos x - 1}$$
A
Step 7
B
Step 6
C
Step 3
D
Step 8
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