Choose a valid substitution for a square-root estimate

Expand $\sqrt{1-x}/(1+2x)$ through $x^2$.4 marks

(i) A student substitutes $x=-1$ to estimate $\sqrt2$. Explain why this is wrong.1 mark

(ii) Put $x=1/9$ in the expansion to show $\sqrt2\approx p/1296$, and find the positive integer $p$.2 marks