Difficulty using "symsum" within a function definition

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Riacon12 2018-4-27

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https://ww2.mathworks.cn/matlabcentral/answers/397824-difficulty-using-symsum-within-a-function-definition

评论： Riacon12 2018-4-27

I'm trying to define a function "ansin" that takes in an array of test values and outputs the function values of the n-th degree Taylor polynomial for sin(x). I am totally unfamiliar with symsum and just tried to do the best I could to copy the notation in the MatLab documentation . Below is the summation that I'm trying to emulate, the code I used and the error I got.

The only difference between the formula and my code is that I've replaced all the n's with k's so that "n" can stand for the degree of taylor polynomial I want. Also I am not summing to infinity but to "n".

testcases = -2*pi + (4*pi).*rand(2000,1);
pred1 = ansin(testcases,1);
pred2 = ansin(testcases,2);
pred3 = ansin(testcases,3);
pred4 = ansin(testcases,4);
function y = ansin(x,n)
   y = symsum(((-1)^(k-1))*((x^(2*k-1))/factorial(2*k-1)), k, 1, n);
end

Undefined function or variable 'k'.

Error in Taylor_Polynomial_Practice>ansin (line 10) y = symsum(((-1)^(k-1))*((x^(2*k-1))/factorial(2*k-1)), k, 1, n);

Error in Taylor_Polynomial_Practice (line 2) pred1 = ansin(testcases,1);

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Riacon12 2018-4-27

I'd also like to add that I know there is taylor series functionality right in Matlab, but I'm using this example to get some experience with the symsum command.

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