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Differential Equations

Question
CBSEENMA12033125

Show that the given differential equation is homogeneous and solve it:
straight x squared dy over dx space equals space straight x squared minus 2 straight y squared plus straight x space straight y





Solution
The given differential equation is
            space straight x squared dy over dx space equals space straight x squared minus 2 straight y squared plus xy
or     dy over dx space equals fraction numerator straight x squared minus 2 straight y squared plus straight x space straight y over denominator straight x squared end fraction                            ...(1)
It is a differential equation of the form dy over dx space equals space straight F left parenthesis straight x comma space straight y right parenthesis
Here,       straight F left parenthesis straight x comma space straight y right parenthesis space equals space fraction numerator straight x squared minus 2 straight y squared plus straight x space straight y over denominator straight x squared end fraction
Replacing x by λx and y by λy, we get.
                straight F left parenthesis λx comma space λy right parenthesis space equals space fraction numerator straight lambda squared straight x squared minus 2 straight lambda squared straight y squared plus straight lambda squared straight x space straight y over denominator straight lambda squared straight x squared end fraction space equals space fraction numerator straight lambda squared left parenthesis straight x squared minus 2 straight y squared plus xy right parenthesis over denominator straight lambda squared straight x squared end fraction space equals space straight lambda degree space left square bracket straight F left parenthesis straight x comma space straight y right parenthesis right square bracket               

∴ F(x, y) is a homogeneous function of degree zero.
∴      given differential equation is a homogeneous differential equation.
Put   y = vx   rightwards double arrow space space dy over dx space equals space straight v. space 1 space space plus straight x space dv over dx
Substituting these values of y and dy over dx in the given equation, we get
                  straight v plus straight x dv over dx space equals space fraction numerator straight x squared minus 2 straight v squared straight x squared plus vx squared over denominator straight x squared end fraction      
or                  straight v plus straight x dv over dx equals 1 minus 2 straight v squared plus straight v space space space space space rightwards double arrow space space space space space straight x dv over dx equals 1 minus 2 straight v squared
rightwards double arrow space space space space space space space space 1 over straight x dx space equals space fraction numerator 1 over denominator 1 minus 2 straight v squared end fraction dv
Integrating, integral 1 over straight x dx space equals space integral fraction numerator 1 over denominator 1 minus 2 straight v squared end fraction dv
therefore space space space space space integral 1 over straight x dx space equals space 1 half integral fraction numerator dv over denominator open parentheses begin display style fraction numerator 1 over denominator square root of 2 end fraction end style close parentheses squared minus straight v squared end fraction
therefore space space space space space log space open vertical bar straight x close vertical bar space space equals 1 half. space fraction numerator 1 over denominator 2. space begin display style fraction numerator 1 over denominator square root of 2 end fraction end style end fraction log space open vertical bar fraction numerator begin display style fraction numerator 1 over denominator square root of 2 end fraction end style plus straight v over denominator begin display style fraction numerator 1 over denominator square root of 2 end fraction end style minus straight v end fraction close vertical bar plus straight c
rightwards double arrow space space space space space space space log space open vertical bar straight x close vertical bar space equals space fraction numerator 1 over denominator 2 square root of 2 end fraction space log space space open vertical bar fraction numerator 1 plus square root of 2 space straight v over denominator 1 minus square root of 2 straight v end fraction close vertical bar plus straight c
rightwards double arrow space space space log space open vertical bar straight x close vertical bar space equals space fraction numerator 1 over denominator 2 square root of 2 end fraction log space open vertical bar fraction numerator 1 plus square root of 2. space begin display style straight y over straight x end style over denominator 1 minus square root of 2. begin display style straight y over straight x end style end fraction close vertical bar plus straight c
rightwards double arrow space space space space log space open vertical bar straight x close vertical bar space equals space fraction numerator 1 over denominator 2 square root of 2 end fraction log space open vertical bar fraction numerator straight x plus square root of 2 straight y over denominator straight x minus square root of 2 straight y end fraction close vertical bar plus straight c

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