Đáp án:
c) `frac{2}{sqrt{5}+2}+frac{2}{sqrt{5}-2}`
`=frac{2(sqrt{5}-2)}{1}+frac{2(sqrt{5}+2)}{1}`
`=2(sqrt{5}-2)+2(sqrt{5}+2)`
`=2sqrt{5}-4+2sqrt{5}+4`
`=4sqrt{5}`
d) `sqrt{13+30sqrt{2+sqrt{9+4sqrt{2}}}`
`=sqrt{13+30sqrt{2+sqrt{1+4sqrt{2}+8}}}`
`=sqrt{13+30sqrt{2+sqrt{(1+sqrt{8})²}}}`
`=sqrt{13+30sqrt{2+1+sqrt{8}}}`
`=sqrt{13+30sqrt{(sqrt{2}+1)²}}`
`=sqrt{13+30(sqrt{2}+1)}`
`=sqrt{13+30sqrt{2}+30}`
`=sqrt{43+30sqrt{2}}`
`=sqrt{25+2.5.3sqrt{2}+18}`
`=sqrt{(5+3sqrt{2})²}`
`=5+3sqrt{2}`
$\text{Shield Knight}$
