Home page for accesible maths Math 101 Chapter 4: Taylor series and complex numbers

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4.8 The Maclaurin series for sine

Example

The Maclaurin series of sinx\sin x\,.

Solution. We calculate the successive derivatives in left-hand column and their values at x=0x=0\, in the right-hand columns:

f(x)=sinx,      f(0)=0;f(x)=\sin x,\qquad\qquad\qquad f(0)=0;\qquad\qquad
f(x)=cosx      f(0)=1;f^{\prime}(x)=\cos x\qquad\qquad\qquad f^{\prime}(0)=1;\qquad\qquad
f′′(x)=-sinx   f′′(0)=0;f^{\prime\prime}(x)=-\sin x\quad\qquad\qquad f^{\prime\prime}(0)=0;\qquad\qquad
f′′′(x)=-cosx   f′′′(0)=-1;f^{\prime\prime\prime}(x)=-\cos x\quad\qquad\qquad f^{\prime\prime\prime}(0)=-% 1;\qquad\qquad
f(4)(x)=sinx      f(4)(0)=0;f^{(4)}(x)=\sin x\qquad\qquad\qquad f^{(4)}(0)=0;\qquad\qquad

and the pattern repeats with a period of four; hence only odd powers appear and the signs alternate. We feed the right-hand coefficients into the general Maclaurin formula to get