MATLAB: Integrating and substituting

integrationSymbolic Math Toolbox

So I have a long code and now I want to differentiate a formula, y, with respect to x in and integral 0 to t. However, I want to keep the previous x for more calculation and substitute my x. Howe do I do this? Example:

x=5;
syms x
int(y,x,0,t)

I'm convinced I keep to substitue, using sub(), but I am unsure how tis would work out because x is now a plain variable (from syms). Any ideas?

Best Answer

Write y in terms of a different symbol, such as X, and

subs(y, X, sym('x'))

to get y in terms of the symbol x (distinct from your variable "x")

Or is y a string instead of a symbolic expression? If it is, then you should be able to do

int(y, 'x', 0, t)

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Use subs() to see the substituted values

syms b x
y = 20*power(.05,b)*int(power(sin(x),b+1),x,0,18.19);
for i= -1:.1:0
    subs(y, b, i)
end

Or save them in an array

syms b x
y = 20*power(.05,b)*int(power(sin(x),b+1),x,0,18.19);
B = -1:.1:0;
z = zeros(size(B), 'like', b)';
for i = 1:numel(B)
    z(i) = subs(y, b, B(i));
end

You can also convert to double data type to see the numeric values

syms b x
y = 20*power(.05,b)*int(power(sin(x),b+1),x,0,18.19);
for i= -1:.1:0
    double(subs(y, b, i))
end

Best Answer

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