`2\sqrt{5}+\sqrt{(2-\sqrt{5})^2}`
`= 2\sqrt{5}+\sqrt{5}-2`
`= 3\sqrt{5}-2`
$\\$
`a. = 5+2.\sqrt{5}.2+4=(\sqrt{5})^2+4\sqrt{5}+2^2=(\sqrt{5}+2)^2`
`b. = 16-2.\sqrt{7}.4+7=4^2-8\sqrt{7}+(\sqrt{7})^2=(4-\sqrt{7})^2`
`c. = 3-2.\sqrt{3}.1+1=(\sqrt{3})^2-2\sqrt{3}+1^2=(\sqrt{3}-1)^2`
`d. = 9+2.\sqrt{3}.3+3=3^2+6\sqrt{3}+(\sqrt{3})^2=(3+\sqrt{3})^2`
(Mik thấy câu d ko hợp lí nên xin sửa)