Binomial expansion....

Use the binomial expansion of (1+x)^(-1/2) to find an approximation for 1/(rt4.2).

I've got to (1+x)^(-1/2) easily, but the obvious idea of substituting x=3.2 gives me the wrong answer. I think it's something to do with the expansion being valid but can't remember. Any help would be much appreciated.
Love, Crappy Math Student.xxx

oh yeah, and the expansion is (1+x)^(-1/2)= 1-(1/2)x+(3/8)x^2...

i think x<2 for it to be valid. not sure how to answer your question though...

bump

The binomial expansion is basically a Taylor expansion of 1/sqrt(x+1) about x=0.  However, this function has a branch point at x = -1, so the series expansion diverges for all |x| > 1.

You could always factor something out to make the expression not phail.
1/sqrt(4.2) = 1/sqrt(4*1.05) = 1/2sqrt(1.05) = 1/2sqrt(1 + .05)