- #1
Astro
- 48
- 1
Wondering why getting different values of "C" depending on how I solve the question. Not sure the values are different. Thanks.
1. Homework Statement
Solve the initial value problem [itex]cos(x)Ln(y) \frac{dy} {dx} =ysin(x) [/itex], y>0, y(0)=e2.
N/A.
[itex]∫ \frac{Ln(y)} {y} dy=∫ \frac{sin x} {cos x} dx [/itex]
[itex]∫ \frac{u} {y} y du=∫ tan(x) dx [/itex]
## \frac{1} {2}u^2+C_1=Ln|sec(x)| + C_2 ##
##\frac{1} {2}Ln^2 (y)=Ln|sec(x)| + C_3 ## ,Since ## C=C_2-C_1 ##
## Ln(y)=± \sqrt{2 Ln|sec(x)|+C} ## Equation #1
## y=e^{± √(2Ln|sec(x)|+C)} ##
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## e^2=e^{+√(2Ln|sec(x)|+C)} ## Since y(0)=e2. Also, exponent cannot be -ve since LS.=RS.
## e^2=e^{+√(2(0)+C)} ##
##2=√C ##
## C=4 ##
Therefore ## y=e^{± √(2Ln|sec(x)|+4)} ## <--- ANS #1
However, if we try solving for C using eqn #1:
## \frac{1}{2}Ln^2(y)=Ln|sec(x)|+C ##
## \frac{1}{2}Ln^2(e^2)=Ln|sec(0)|+C ##
## \frac{1}{2}(2^2)Ln^2(e)=Ln(1)+C ##
## 2(1)=0+C ##
Therefore, ## y=e^{± √(2Ln|sec(x)|+2)} ## <----ANS #2.
However, obviously ANS #1 ≠ ANS #2. Can someone please help me understand why and what I did wrong?
Thanks.
1. Homework Statement
Solve the initial value problem [itex]cos(x)Ln(y) \frac{dy} {dx} =ysin(x) [/itex], y>0, y(0)=e2.
Homework Equations
N/A.
The Attempt at a Solution
[itex]∫ \frac{Ln(y)} {y} dy=∫ \frac{sin x} {cos x} dx [/itex]
u=Ln(y)
[itex]du=\frac{1} {y}[/itex] dy
dy=y du
[itex]du=\frac{1} {y}[/itex] dy
dy=y du
[itex]∫ \frac{u} {y} y du=∫ tan(x) dx [/itex]
## \frac{1} {2}u^2+C_1=Ln|sec(x)| + C_2 ##
##\frac{1} {2}Ln^2 (y)=Ln|sec(x)| + C_3 ## ,Since ## C=C_2-C_1 ##
## Ln(y)=± \sqrt{2 Ln|sec(x)|+C} ## Equation #1
## y=e^{± √(2Ln|sec(x)|+C)} ##
--------------
## e^2=e^{+√(2Ln|sec(x)|+C)} ## Since y(0)=e2. Also, exponent cannot be -ve since LS.=RS.
## e^2=e^{+√(2(0)+C)} ##
##2=√C ##
## C=4 ##
Therefore ## y=e^{± √(2Ln|sec(x)|+4)} ## <--- ANS #1
However, if we try solving for C using eqn #1:
## \frac{1}{2}Ln^2(y)=Ln|sec(x)|+C ##
## \frac{1}{2}Ln^2(e^2)=Ln|sec(0)|+C ##
## \frac{1}{2}(2^2)Ln^2(e)=Ln(1)+C ##
## 2(1)=0+C ##
Therefore, ## y=e^{± √(2Ln|sec(x)|+2)} ## <----ANS #2.
However, obviously ANS #1 ≠ ANS #2. Can someone please help me understand why and what I did wrong?
Thanks.
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