Hugh Soto
2022-09-12
Answered

What is a particular solution to the differential equation $\frac{dy}{dx}=\frac{4\sqrt{y}\mathrm{ln}x}{x}$ with y(e)=1?

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asked 2022-07-14

A car is travelling at 100 km/h on a level road when it runs out of fuel. Its speed v (in km/h) starts to decrease according to the formula

$\frac{dv}{dt}=-kv\phantom{\rule{1em}{0ex}}(1)$

where k is constant. One kilometre after running out of fuel its speed has fallen to 50 km/h. Use the chain rule substitution

$\frac{dv}{dt}=\frac{dv}{ds}\frac{ds}{dt}=\frac{dv}{ds}v$

to solve the differential equation.

Note: Although I haven't solved it yet, the answers say that this isn't a reasonable model as the velocity is always positive; I didn't make a typo in the question.

What I'm trying to do is solve velocity as a function of displacement (s, in km), velocity as a function of time (t, in hours), and displacement as a function of time (I need these functions for later parts of the question).

So far I've found velocity as a function of displacement (v(s)):

$\frac{dv}{dt}=-k\frac{ds}{dt}\phantom{\rule{1em}{0ex}}\text{(from (1))}$

$\int \frac{dv}{dt}dt=-k\int \frac{ds}{dt}dt$

$v(s)=-ks+C$

$v(0)=100\phantom{\rule{thickmathspace}{0ex}}\u27f9\phantom{\rule{thickmathspace}{0ex}}C=100,\text{}v(1)=50\phantom{\rule{thickmathspace}{0ex}}\u27f9\phantom{\rule{thickmathspace}{0ex}}k=50$

$v(s)=-50s+100$

Then I've tried to find velocity as a function of time (v(t)), but I've got stuck. I can't find any differential equation I can use to get this, or to get displacement as a function of time (s(t)).

The answer key says $v(t)=100{e}^{-50t}$ and $s(t)=2(1-{e}^{-50t})$

I've solved such questions many times before, but it's been a while so I'm a bit rusty. So, even a hint might be enough for me to realise what to do.

$\frac{dv}{dt}=-kv\phantom{\rule{1em}{0ex}}(1)$

where k is constant. One kilometre after running out of fuel its speed has fallen to 50 km/h. Use the chain rule substitution

$\frac{dv}{dt}=\frac{dv}{ds}\frac{ds}{dt}=\frac{dv}{ds}v$

to solve the differential equation.

Note: Although I haven't solved it yet, the answers say that this isn't a reasonable model as the velocity is always positive; I didn't make a typo in the question.

What I'm trying to do is solve velocity as a function of displacement (s, in km), velocity as a function of time (t, in hours), and displacement as a function of time (I need these functions for later parts of the question).

So far I've found velocity as a function of displacement (v(s)):

$\frac{dv}{dt}=-k\frac{ds}{dt}\phantom{\rule{1em}{0ex}}\text{(from (1))}$

$\int \frac{dv}{dt}dt=-k\int \frac{ds}{dt}dt$

$v(s)=-ks+C$

$v(0)=100\phantom{\rule{thickmathspace}{0ex}}\u27f9\phantom{\rule{thickmathspace}{0ex}}C=100,\text{}v(1)=50\phantom{\rule{thickmathspace}{0ex}}\u27f9\phantom{\rule{thickmathspace}{0ex}}k=50$

$v(s)=-50s+100$

Then I've tried to find velocity as a function of time (v(t)), but I've got stuck. I can't find any differential equation I can use to get this, or to get displacement as a function of time (s(t)).

The answer key says $v(t)=100{e}^{-50t}$ and $s(t)=2(1-{e}^{-50t})$

I've solved such questions many times before, but it's been a while so I'm a bit rusty. So, even a hint might be enough for me to realise what to do.

asked 2022-09-08

What is a solution to the differential equation $\frac{dy}{dx}=3y$?

asked 2022-01-19

Determine if the equation is linear or non-linear and indicate the order of the differential equation and would you tell me why please

$(2{x}^{3}+y)dx-xdy=0$

$ydx-(x+6{y}^{2})dy=0$

$(x+y)dx+\mathrm{tan}\left(x\right)dy=0$

$6xydx+(4y+9{x}^{2})dy=0$

asked 2022-05-21

My Problem is this given System of differential Equations:

$\dot{x}=8x+18y$

$\dot{y}=-3x-7y$

I am looking for a gerenal solution.

My Approach was: i can see this is a System of linear and ordinary differential equations. Both are of first-order, because the highest derivative is the first. But now i am stuck, i have no idea how to solve it. A Transformation into a Matrix should lead to this expression:

$\overrightarrow{y}=\left(\begin{array}{cc}8& 18\\ -3& -7\end{array}\right)\cdot x$

or is this correct:

$\overrightarrow{x}=\left(\begin{array}{cc}8& 18\\ -3& -7\end{array}\right)\cdot y\text{?}$

But i don't know how to determine the solution, from this point on.

$\dot{x}=8x+18y$

$\dot{y}=-3x-7y$

I am looking for a gerenal solution.

My Approach was: i can see this is a System of linear and ordinary differential equations. Both are of first-order, because the highest derivative is the first. But now i am stuck, i have no idea how to solve it. A Transformation into a Matrix should lead to this expression:

$\overrightarrow{y}=\left(\begin{array}{cc}8& 18\\ -3& -7\end{array}\right)\cdot x$

or is this correct:

$\overrightarrow{x}=\left(\begin{array}{cc}8& 18\\ -3& -7\end{array}\right)\cdot y\text{?}$

But i don't know how to determine the solution, from this point on.

asked 2020-11-08

Solve differential equation$x{y}^{\prime}=(1-{y}^{2}{)}^{\frac{1}{2}}$

asked 2021-02-20

Solve differential equation
$dy/dx=({y}^{2}-1)/({x}^{2}-1)$

asked 2020-11-08

Given the function

Express f(t) in terms of the shifted unit step function u(t -a)

F(t) - ?

Now find the Laplace transform F(s) of f(t)

F(s) - ?