Problem in putting limit to the variable

Question

Ipshita 2020-11-27

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/665218-problem-in-putting-limit-to-the-variable

回答： Ameer Hamza 2020-11-27

I want to find the maximum value of f for x between 0 and 0.9. Please tell how can I put the limit to x in this code.

syms x
0<x & x<0.9;
f=0.3*(x-0.6)*(x-0.9)-0.9*cos(0.6)*x*(x-0.9)+0.6*cos(0.9)*x*(x-0.6)-cos(x);
f2 = diff(f,x)==0;
extreme_points = vpasolve(f2==0,x);
extreme_values = subs(f, x, extreme_points);
extreme_values 

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Answer 1

Ameer Hamza 2020-11-27

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/665218-problem-in-putting-limit-to-the-variable#answer_557818

在 MATLAB Online 中打开

'x' is already in this range. You are printing the value of 'extreme_values', not 'extreme_points'

syms x
f=0.3*(x-0.6)*(x-0.9)-0.9*cos(0.6)*x*(x-0.9)+0.6*cos(0.9)*x*(x-0.6)-cos(x);
f2 = diff(f,x)==0;
extreme_points = vpasolve(f2==0,x);
extreme_values = subs(f, x, extreme_points);

Result

>> extreme_points
extreme_points =
0.0061113661628718004024766518326605 % greater than 0
>> extreme_values
extreme_values =
-0.83801606593057221627803628819248

In general, you can specify the range in vpasolve like this

extreme_points = vpasolve(f2==0,x, [0 0.9]);