$\sqrt{2+2cosa}=\sqrt{2(cosa+1)}=\sqrt{2.2cos^2\frac{a}{2}}=2cos\frac{a}{2}$ ( Có $1$ dấu căn)$\sqrt{2+\sqrt{2+2cosa}}=\sqrt{2+2cos\frac{a}{2}}=2cos\frac{a}{4}$ ( Có $2$ dấu căn)
...
$\sqrt{2+\sqrt{2+...+\sqrt{2+2cosa}}}=2cos\frac{a}{2^n}$ ( Có $n$ dấu căn )