Question 2945
 Taylor series was one of the importance achievement of human science.
 
 We tried to get the approximation for an arbitray function by
 polynomials. Since, with polynomials, we know how todifferentiate,
 integrate, find the roots,draw the graphs,etc.

 The results of Taylor's series is,
 for a nth differentiable function  f(x)
        oo
 f(x) = E f^(k)(a)(x-a)^k/k! ,         
       k=o
 (the series converges to f(x) locally at a)

 when a = 0, we have

         oo
 f(x) = E f^(k)(0)x^k/k! 
        k=o

 Without this series, we cannot use any calculators or computers
 to find the value of sine,cosine,log or expontial, ect.
 In other words, there is no modern science without Taylor series.

 Now, f(x) = sin x, expand it at x = 0,
 we have f'(x) = cos x, f'(x) = -sin x, f"'(x) = -cos x,
 so f'(0) = 1, f"(0) = 0, f"'(0) = -1,

 after simplified, we have
         oo
 sin x = E (-1)^(2k+1))x^(2k+1)/(2k+1)! 
        k=o

 or sin x = x -x^3/3! + x^5 /5! -x^7/7! +....

 I used the first two terms to estimate sinx before.
 You also can do search to get more informations from the Internet.

 Good luck.

 Kenny