I have a linear first order ordinary differential equation d y </mrow

Cory Patrick

Cory Patrick

Answered question

2022-06-22

I have a linear first order ordinary differential equation
d y d x + tan ( x ) y = 2 cos 2 x sin x sec x
with an initial condition as y ( π 4 ) = 3 2
My integrating factor μ ( x ) = sec x
After multiplication with the integration factor what I get is:
( s e c x   y ) = s i n 2 x s e c 2 x
or
( s e c x   y ) = 2 s i n x c o s x s e c 2 x
If I use the first equation I get:
y ( x ) = 1 2 c o s 2 x t a n x + c s e c x
and using the second equation I get:
y ( x ) = s i n 2 x t a n x + c s e c x
2   s i n x   c o s x   d x = 2 s i n 2 x 2 = s i n 2 x
with the first equation I get c=7 and second equation I get c = 13 2 .
It is a very simple differential equation but when I solve it I get two different answers. Is this ok?

Answer & Explanation

Esteban Johnson

Esteban Johnson

Beginner2022-06-23Added 15 answers

The primitive of sin ( 2 x ) is 1 2 cos ( 2 x ). Then the difference in the integration constants c is just the constant term in the double-angle formula cos ( 2 x ) = 1 2 sin 2 x

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