Use the binomial expansion of (1+x)^(-1/2) to find an approximation for 1/(rt4.2).
I've got the expansion of (1+x)^(-1/2) as 1-(1/2)x+(3/8)x^2...
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...