Function & Plot?

Question

Abdullah Azzam 2015-10-17

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https://ww2.mathworks.cn/matlabcentral/answers/249133-function-plot

评论： Steven Lord 2015-10-17

I am trying to solve an example that wants me to solve the equation(c=c-(k*c*t)) and plot the graph between c and t, knowing that t is the step size which in my case is 0.15, and k is constant=0.3, and c=15 initially, I am looking to create a function that calculate the decaying rate with time I have tried the following code: function Decayrate ()

k=0.3;

c=15;

t=(0:0.15:2.1);

c=c-(k*c*t)

plot (t,c)

but the problem is that it when applying the equation the t that going to be used is the the t value and not the step size (0.15), so how can I apply the step size instead of t to the equation, and plot c with t at the same time knowing that at t=0 c=15?

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Eng. Fredius Magige 2015-10-17

Hi t=0, c=0.15 Noted that equation c is implicit? I think you have to use while function as long you known the boundary, (how far it has to go!!!!!!)

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Answer 1

Martin Schätz 2015-10-17

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From what i did understand, if c=15 initialy, you have to change it every time. so the newc=c-(k*c*t) and also t will change every time. So i thing the code should look like this:

clear all
k=0.3;
c=15;
t=(0:0.15:2.1);
c=zeros(1,15)

Setup initial c to 15

c(1)=15;

for every next c, i will use the previous c and right t

for i=2:15
  c(i)=c(i-1)-(k*c(i-1)*t(i))
end
figure
plot(t,c), axis tight
xlabel('t')
ylabel('c')

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Abdullah Azzam 2015-10-17

guess I couldn't explain the question briefly, let me try again, I have the following equation that I want to solve numerically, dc/dt=-kc then plot the graph between c and t, in your answer c hit the 0 by the time t = 2.1 sec while it should almost be 8.12, is it clear that way?