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Ex 9.4, 4 show that the given differential equation is homogeneous and solve each of them. (𝑥^2−𝑦^2 )𝑑𝑥+2𝑥𝑦 𝑑𝑦=0 Step 1: Find 𝑑𝑦/𝑑𝑥 (𝑥^2−𝑦^2 )𝑑𝑥+2𝑥𝑦 𝑑𝑦=0 2xy dy = − (𝑥^2−𝑦^2 ) dx 2xy dy = (𝑦^2−𝑥^2 ) dx 𝒅𝒚/𝒅𝒙 = (𝒚^𝟐 − 𝒙^𝟐)/𝟐𝒙𝒚 Step 2: Putting F(x, y) = 𝑑𝑦/𝑑𝑥 and finding F(𝜆x, 𝜆y) F(x, y) = (𝑦^2 − 𝑥^2)/2𝑥𝑦 F(𝜆x, 𝜆y) = ((𝜆𝑦)^2−(𝜆𝑥)^2)/(2𝜆𝑥. 𝜆𝑦)= (𝑥^2 𝑦^2 −𝜆^2 𝑥^2)/(𝜆^2.2𝑥𝑦)= (𝜆^2 (𝑦^2 − 𝑥^2))/(𝜆^2.2𝑥𝑦) = (𝑦^2 −𝑥^2)/2𝑥𝑦 = F(x, y) ∴ F(𝜆x, 𝜆y) = F(x, y) = 𝜆° F(x, y) Hence, F(x, y) is a homogenous Function of with degree zero So, 𝑑𝑦/𝑑𝑥 is a homogenous differential equation. Step 3: Solving 𝑑𝑦/𝑑𝑥 by putting y = vx Put y = vx. differentiating w.r.t.x 𝑑𝑦/𝑑𝑥 = x 𝑑𝑣/𝑑𝑥+𝑣𝑑𝑥/𝑑𝑥 𝒅𝒚/𝒅𝒙 = 𝒙 𝒅𝒗/𝒅𝒙 + v Putting value of 𝑑𝑦/𝑑𝑥 and y = vx in (1) 𝑑𝑦/𝑑𝑥 = (𝑦^2 − 𝑥^2)/2𝑥𝑦 (𝒙 𝒅𝒗)/𝒅𝒙+𝒗 = (〖(𝒙𝒗)〗^𝟐 − 𝒙^𝟐)/(𝟐𝒙(𝒙 𝒗)) (𝑥 𝑑𝑣)/𝑑𝑥+𝑣 = (𝑥^2 𝑣^2−𝑥^2)/(2𝑥^2 𝑣) (𝑥 𝑑𝑣)/𝑑𝑥 = (𝑥^2 𝑣^2 − 𝑥^2)/(2𝑥^2 𝑣) − v x 𝑑𝑣/𝑑𝑥 = (𝑥^2 𝑣^2 − 𝑥^2 − 2𝑥^2 𝑣^2)/(2𝑥^2 𝑣) x 𝑑𝑣/𝑑𝑥 = (〖−𝑥〗^2 𝑣^2 − 𝑥^2)/(2𝑥^2 𝑣) 𝑑𝑣/𝑑𝑥 = − 1/𝑥 ((𝑥^2 〖(𝑣〗^2 + 1))/(2𝑥^2 𝑣)) 𝑑𝑣/𝑑𝑥 = − 1/𝑥 ((𝑣^2 + 1)/2𝑣) (𝟐𝒗 𝒅𝒗)/(𝒗^𝟐 + 𝟏) = (−𝒅𝒙)/𝒙 Integrating both sides ∫1▒2𝑣/(𝑣^2 + 1) 𝑑𝑣 = ∫1▒(−𝑑𝑥)/𝑥 ∫1▒𝟐𝒗/(𝒗^𝟐 + 𝟏) 𝒅𝒗 = −𝒍𝒐𝒈⁡|𝒙|+𝑪 Putting t = v2 + 1 diff.w.r.t v. 𝑑/𝑑𝑣 (v2 + 1) = 𝑑𝑡/𝑑𝑣 2v = 𝑑𝑡/𝑑𝑣 dv = 𝒅𝒕/𝟐𝒗 Now, From (2) ∫1▒2𝑣/𝑡 𝑑𝑡/2𝑣 = − log|𝑥|+𝑐 (from (2)) ∫1▒𝒅𝒕/𝒕 = − log|𝒙|+𝒄 log|𝑡| = − log|𝑥|+𝑐 Putting t = v2 + 1 log|"v2 + 1" | = –log|"x" |+𝑐 log |"v2 + 1" | + log|"x" |=𝑐 log |"x(v2 + 1)" | = C Putting v = 𝑦/𝑥 log |[(𝑦/𝑥)^2+1]𝑥|=𝑐 log|(𝑦^2 + 𝑥^2)/𝑥^2 ×𝑥|=𝑐 Putting 𝑐 = log c log |(𝒚^𝟐 + 𝒙^𝟐)/𝒙| = log c 𝑦^2+𝑥^2 = cx 𝒙^𝟐+𝒚^𝟐=𝒄𝒙 is the general solution of the given differential equation.

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Davneet Singh

Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 14 years. He provides courses for Maths, Science and Computer Science at Teachoo