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Solve the differential equation dy/dx = 1/(xy) + y/x given that y = √3 when x = 1 - Leaving Cert Applied Maths - Question 10 - 2009

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Solve the differential equation dy/dx = 1/(xy) + y/x given that y = √3 when x = 1. A particle of mass m is projected vertically upwards with speed u. The air resi... show full transcript

Worked Solution & Example Answer:Solve the differential equation dy/dx = 1/(xy) + y/x given that y = √3 when x = 1 - Leaving Cert Applied Maths - Question 10 - 2009

Step 1

Solve the differential equation

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Answer

To solve the differential equation, we start from:

dydx=1xy+yx\frac{dy}{dx} = \frac{1}{xy} + \frac{y}{x}

Rearranging gives us:

dy=(1xy+yx)dxdy = \left(\frac{1}{xy} + \frac{y}{x}\right) dx

Integrating both sides:

y+y2dy=(1x+1xy)dx\int y + y^2 dy = \int \left(\frac{1}{x} + \frac{1}{x} y\right) dx

This results in:

12ln(1+y2)=lnx+C\frac{1}{2} \ln(1+y^2) = \ln x + C

Exponentiating to solve for y, we substitute the initial condition y = √3 when x = 1:

After performing the integration and back-substituting, we find a specific solution for y.

Step 2

Find the value of u in terms of k.

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Answer

Using the equation of motion with air resistance:

mgmk=mdvdt-mg - mk = m\frac{dv}{dt}

Rearranging, we have:

[-\int (\frac{1}{g + kv^2}) dv = \int dt]

Integrating this leads us to find:

u=3gku = \sqrt{\frac{3g}{k}}

Step 3

Find the value of k if the time to reach the greatest height is π/3 seconds.

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Answer

The equation from before leads us to:

1g+kv2dv\int \frac{1}{g + kv^2} dv

By substituting and integrating, we determine:

After solving this time integration for t = π/3, we find:

k=1gk = \frac{1}{g}.

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