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Rakesh tried to solve the differential equation 
(dy)/(dx)=2y*cos(x). This is his work:

(dy)/(dx)=2y*cos(x)
Step 1: 
quad int(1)/(y)dy=int2cos(x)dx
Step 2: 
quad ln |y|=2sin(x)
Step 3: 
quade^(ln |y|)=e^(2sin(x))
Step 4: 
quad|y|=e^(2sin(x))
Step 5: 
quad y=+-e^(2sin(x))+C
Is Rakesh's work correct? If not, what is his mistake?
Choose 1 answer:
(A) Rakesh's work is correct.
(B) Step 1 is incorrect. The separation of variables wasn't done correctly.
(C) Step 2 is incorrect. The right-hand side of the equation should be 
2sin(x)+C.
(D) Step 4 is incorrect. 
e^(ln |y|) isn't equal to 
|y|.

Rakesh tried to solve the differential equation dydx=2ycos(x) \frac{d y}{d x}=2 y \cdot \cos (x) . This is his work:\newlinedydx=2ycos(x) \frac{d y}{d x}=2 y \cdot \cos (x) \newlineStep 11: 1ydy=2cos(x)dx \quad \int \frac{1}{y} d y=\int 2 \cos (x) d x \newlineStep 22: lny=2sin(x) \quad \ln |y|=2 \sin (x) \newlineStep 33: elny=e2sin(x) \quad e^{\ln |y|}=e^{2 \sin (x)} \newlineStep 44: y=e2sin(x) \quad|y|=e^{2 \sin (x)} \newlineStep 55: y=±e2sin(x)+C \quad y= \pm e^{2 \sin (x)}+C \newlineIs Rakesh's work correct? If not, what is his mistake?\newlineChoose 11 answer:\newline(A) Rakesh's work is correct.\newline(B) Step 11 is incorrect. The separation of variables wasn't done correctly.\newline(C) Step 22 is incorrect. The right-hand side of the equation should be 2sin(x)+C 2 \sin (x)+C .\newline(D) Step 44 is incorrect. elny e^{\ln |y|} isn't equal to y |y| .

Full solution

Q. Rakesh tried to solve the differential equation dydx=2ycos(x) \frac{d y}{d x}=2 y \cdot \cos (x) . This is his work:\newlinedydx=2ycos(x) \frac{d y}{d x}=2 y \cdot \cos (x) \newlineStep 11: 1ydy=2cos(x)dx \quad \int \frac{1}{y} d y=\int 2 \cos (x) d x \newlineStep 22: lny=2sin(x) \quad \ln |y|=2 \sin (x) \newlineStep 33: elny=e2sin(x) \quad e^{\ln |y|}=e^{2 \sin (x)} \newlineStep 44: y=e2sin(x) \quad|y|=e^{2 \sin (x)} \newlineStep 55: y=±e2sin(x)+C \quad y= \pm e^{2 \sin (x)}+C \newlineIs Rakesh's work correct? If not, what is his mistake?\newlineChoose 11 answer:\newline(A) Rakesh's work is correct.\newline(B) Step 11 is incorrect. The separation of variables wasn't done correctly.\newline(C) Step 22 is incorrect. The right-hand side of the equation should be 2sin(x)+C 2 \sin (x)+C .\newline(D) Step 44 is incorrect. elny e^{\ln |y|} isn't equal to y |y| .
  1. Separate variables: Separate variables.\newlineRakesh starts by separating the variables yy and xx.\newlinedyy=2cos(x)dx\frac{dy}{y} = 2\cos(x)dx\newlineThis is the correct method for separating variables in a differential equation.
  2. Integrate both sides: Integrate both sides.\newlineRakesh integrates both sides of the equation.\newline(1y)dy=2cos(x)dx\int(\frac{1}{y})\,dy = \int 2\cos(x)\,dx\newlinelny=2sin(x)+C\ln|y| = 2\sin(x) + C\newlineRakesh forgot to include the constant of integration on the right-hand side of the equation.

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