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The Sine, Cosine of x can be computed as follows:

$$\sin(x) = x - \dfrac {x^3}{3!} + \dfrac {x^5}{5!} - \dfrac {x^7}{7!} + \dfrac {x^9}{9!} …$$

$$\cos(x) = 1 - \dfrac {x^2}{2!} + \dfrac {x^4}{4!} - \dfrac {x^6}{6!} + \dfrac {x^8}{8!} …$$

How to compute the Sine and Cosine for given values of $x$ (where $x$ is in radians) using the above series upto 5 terms in as less characters of program?

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1 Answer 1

The sequence is alternating and the general term decreases monotonically. So partial sums ending in positive terms will overestimate the value, while partial sums ending in negative terms will be underestimates.

So if $|x| \le 1$, then using $x - x^3/3! + x^5/5! - x^7/7!$ for $\sin$ will have an error less than $\frac{1}{9!} < 10^{-5}$. Similarly, if we use terms of $\cos$ to the $x^8/8!$ term, we get an error less than $\frac{1}{10!} < 10^{-6}$.

However, if $1 < |x| \le \pi$, then we need more terms to achieve this accuracy. For $|x|$ close to $\pi$, you will need to use terms until $x^{12}/{12!}$ for $\cos$ and $x^{13}/13!$ for sine to achieve an accuracy of $5$ decimal places.

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