summation of sinx using summation

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Abraham Robledo 2019-2-21

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回答： James Tursa 2019-2-22

I'm trying to find the value of ???(?) if ? = ? /2 using the following equation. Including 10, 20, 30, and 100 terms in the summation. As such ???(?) = ? −?^3/!+?^5 /5!−x^7/7!... I tried the following but its not working. The numbers are only odd indices. Please help

n = 10
 x = pi/2;
 y = zeros(1,n);
 for i = 0:n
    y(i+1) = (x^(i+1)/factorial(i+1));
 end
 SINx = sum(y) 

=3.8105

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Walter Roberson 2019-2-22

0:n produces all of the indices, not just the odd indices. The odd indices would be 1:2:n when n is the final index, or 1:2:(2*n-1) to include n terms

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Answer 1

James Tursa 2019-2-22

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You are missing the alternating signs of the terms. E.g., you could put in a factor of (-1)^something to get this effect. The "something" would be dependent on the loop index. Also, as Walter points out, you need the indexing for the powers of x and the factorial to step by 2, not step by 1.