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Solve the equation. dy dx = ay + b cy + d , where a, b, c, and d are constants. (assume a ≠ 0 and ay + b ≠ 0.)

User Yos
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1 Answer

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It solves it in x. the solution for y includes heavy use of the product log function.

dy/dx = ay + b/cy +d

(cy +d/ ay + b) dy = dx

∫ (cy +d/ ay + b) dy = x (t) + C

Into solving the integral, integration by parts followed by u substitution and another integration by parts.

∫ (cy +d/ ay + b) dy

u = cy + d dv = dy/ay + b

du = c dy v = ln I ay + b l / a

Then, use u substitution for the new integral

u = ay + b

du = a dy

∫ ln l ay + b I dy = ∫ ln IuI /a du = 1/a ∫ ln luI du

Integrating the natural log includes thus far another integration by parts

r = ln IuI ds = du

dr = du / u (s) = du

∫ ln IuI / du = u ln IuI - ∫ du = u ln IuI - ∫ a dy = (ay + b) ln Iay +bl – ay

The original integral of expression

∫ (cy +d/ ay + b) dy = cy + d/a ln lay+bl – c/a² [(ay+b) ln lay+bl – ay]

Then simplify

∫ (cy +d/ ay + b) dy = cy + d/a ln lay+bl – c/a²[(ay+b) ln lay+bl – ay]

= a (cy + d)/a² ln lay+bl – c (ay+b)/ a²ln lay+bl + c/a² ay

= cay + ad – cay – cb/ a² ln lay+bl + cay/a²

= ad – cb/a²ln lay+bl + cy/a

The final answer will be

x(t) + C = ad – cb/a² ln lay+bl + cy/a

x(t) = ad – cb/a² ln lay+bl + cy/a + k

User Dfowj
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