Binomial expansion (C4)

The question asks me to find the first 4 terms of (1-1/x)^1/2
Which I had found and the book stated to be correct
1-1/2x -`1/8x^2 - 1/16x^3
Then it asks me to replace x with 100 to find the root of 99.

based on the top expression i assumed that this would be
root99/10 so i had to multiply by 10 at the end
however Im getting the value 0.9486875x10. Which is not the root of 99 nor is it written in the answer.
Since this really should work i must have gone wrong somewhere?

The question asks me to find the first 4 terms of (1-1/x)^1/2
Which I had found and the book stated to be correct
1-1/2x -`1/8x^2 - 1/16x^3
Then it asks me to replace x with 100 to find the root of 99.

based on the top expression i assumed that this would be
root99/10 so i had to multiply by 10 at the end
however Im getting the value 0.9486875x10. Which is not the root of 99 nor is it written in the answer.
Since this really should work i must have gone wrong somewhere?