2—Infinite Series 25Of course, even better than memorizing them is to understand their derivations so well that you
can derive them as fast as you can write them down. For example, the cosine is the derivative of the
sine, so if you know the latter series all you have to do is to differentiate it term by term to get the
cosine series. The logarithm of(1 +x)is an integral of 1 /(1 +x)so you can get its series from that
of the geometric series. The geometric series is a special case of the binomial series forα=− 1 , but
it’s easier to remember the simple case separately. You can express all of them as special cases of the
general Taylor series.
What is the sine of 0. 1 radians? Just use the series for the sine and you have the answer, 0.1, or
to more accuracy, 0. 1 − 0. 001 /6 = 0. 099833
What is the square root of 1.1?√
1 .1 = (1 +.1)^1 /^2 = 1 +^12. 0 .1 = 1. 05
What is 1/1.9? 1 /(2−.1) = 1/[2(1−.05)] =^12 (1 +.05) =.5 +.025 =. 525 from the first
terms of the geometric series.
What is^3
√
1024?^3
√
1024 =^3
√
1000 + 24 =^3
√
1000(1 + 24/1000) =
10(1 + 24/1000)^1 /^3 = 10(1 + 8/1000) = 10. 08
As you see from the last two examples you have to cast the problem into a form fitting the
expansion that you know. When you want to use the binomial series, rearrange and factor your expression
so that you have (
1 +something small
)α2.2 Deriving Taylor Series
How do you derive these series? The simplest way to get any of them is to assume that such a series
exists and then to deduce its coefficients in sequence. Take the sine for example, assume that you can
write
sinx=A+Bx+Cx^2 +Dx^3 +Ex^4 +···
Evaluate this atx= 0to get
sin 0 = 0 =A+B0 +C 02 +D 03 +E 04 +···=A
so the first term,A= 0. Now differentiate the series, getting
cosx=B+ 2Cx+ 3Dx^2 + 4Ex^3 +···
Again setx= 0and all the terms on the right except the first one vanish.
cos 0 = 1 =B+ 2C0 + 3D 02 + 4E 03 +···=B
Keep repeating this process, evaluating in turn all the coefficients of the assumed series.
sinx=A+Bx+Cx^2 +Dx^3 +Ex^4 +···
cosx=B+ 2Cx+ 3Dx^2 + 4Ex^3 +···
−sinx= 2C+ 6Dx+ 12Ex^2 +···
−cosx= 6D+ 24Ex+ 60Fx^2 +···
sinx= 24E+ 120Fx+···
cosx= 120F+···
sin 0 = 0 =A
cos 0 = 1 =B
−sin 0 = 0 = 2C
−cos 0 =−1 = 6D
sin 0 = 0 = 24E
cos 0 = 1 = 120F
This shows the terms of the series for the sine as in Eq. (2.4).
Does this show that the series converges? If it converges does it show that it converges to the
sine? No to both. Each statement requires more work, and I’ll leave the second one to advanced