help me to find max :((

syms x;
f(x) = (2.*x-1).*sin(pi.*x);
xMin = -2; xMax = 1; stepSize = 0.01;
for i=1:5
    y=diff(f(x),i);
    [val,idx] = max(y) ;%Error using sym/max (line 101)
                        %Input arguments must be convertible to floating-point numbers.
    eval(val)
end

help me fix it plz

[Image Analyst]

You're using a symbolic x but trying to find the max numerically. Don't use syms. Use linspace to define a range for x, then use max(y), not a for loop, to find the first max. What is your desired range for x?\

x = linspace(-15, 15, 1000)
fx = (2.*x-1).*sin(pi.*x);
plot(x, fx, 'b-');
grid on;
xlabel('x', 'FontSize', fontSize);
ylabel('f(x)', 'FontSize', fontSize)

If you want the max of the derivative, why not take the analytical derivative (by the formula) of the function and find the max of that?

[Phuc Nguyen Quy]

assume(x, 'Real');

I just want to find max of every diff(f,i) with i=1->5

if this way not to good can u tell me another way?

thank you <3

Answer 1

[Stephan]

Hi,

two steps. Symbolic finding the derivatives and then calculate numeric over the functions you got.

syms x y;
f(x) = (2.*x-1).*sin(pi.*x);
for i=1:5
    y(i)=diff(f(x),i);
end
fun=matlabFunction(y');
x=-2:0.01:1;
result=fun(x);
maxres = max(result,[],2)

Best regards

Stephan

[Walter Roberson]

? Is the question to find which of the first 5 derivatives and which location in range x together lead to the maximum value?? Or the maximum over a given range of x for each of the first 5 derivatives? Or the global maximum for each of the first 5 derivatives?

[Phuc Nguyen Quy]

thank you so much <3

[Walter Roberson]

For that function, the global maxima for each of the derivatives can be fairly far outside the range -2 to +1. Indeed, it is not difficult to show that the 4th derivative (for example) has local maxima that increase without bound towards +/- infinity.