
power series
hi :)
I know the power series representation of sin x is Sum [ (1)^n*(x)^(2n+1)/(2n+1)!] but what about (sinx)^2?
another quick question :
give an example of set A that has no supremum and why does it not violate the Completeness Axiom.
is {tanx : 0 < x < pi/2} correct?

To get the power series expansion of $\displaystyle sin^2(x) $, you can do 2 things :
1. Compute and evaluate the derivative up to some order and try to find a general expression for the coefficients;
2. $\displaystyle sin^2(x) = \sum_{i=0}^\infty c_nx^n = \left( \sum_{i=0}^\infty (1)^{n} \frac{x^{2n+1}}{(2n+1)!} \right)^2$
Equate coefficient on both sides using this formula
$\displaystyle
\left(\sum_{n=0}^\infty a_n x^n\right)\left(\sum_{n=0}^\infty b_n x^n\right) $
$\displaystyle = \sum_{i=0}^\infty \sum_{j=0}^\infty a_i b_j x^{i+j} $
$\displaystyle = \sum_{n=0}^\infty \left(\sum_{i=0}^n a_i b_{ni}\right) x^n $
I suggest the second method.

A third way is the use of identity...
$\displaystyle \sin^{2} x = \frac{1}{2}  \frac{1}{2}\cdot \cos 2x$ (1)
... so that is...
$\displaystyle \sin^{2} x = \frac{1}{2} \cdot \sum_{n=1}^{\infty} (1)^{n1} \frac{(2x)^{2n}}{(2n)!}$ (2)
Kind regards
$\displaystyle \chi$ $\displaystyle \sigma$

chisigma's trick is even better

lol why didn't I think of that!
thank you!!! (Rofl)