a, Thay x= 5 vào, ta có:
A= $\sqrt[]{5+2.\sqrt[]{5-1}}$+$\sqrt[]{5-2.\sqrt[]{5-1}}$
= $\sqrt[]{5+2.\sqrt[]{4}}$+$\sqrt[]{5-2.\sqrt[]{4}}$
= $\sqrt[]{5+2.2}$+$\sqrt[]{5-2.2}$
= $\sqrt[]{9}$+$\sqrt[]{1}$
= $3+1=4$
b, A= $\sqrt[]{x+2.\sqrt[]{x-1}}$+$\sqrt[]{x-2.\sqrt[]{x-1}}$
= $\sqrt[]{(x-1)+2.\sqrt[]{x-1}+1}$+$\sqrt[]{(x-1)-2.\sqrt[]{x-1}+1}$
= $\sqrt[]{(\sqrt[]{x-1}+1)²}$+$\sqrt[]{(\sqrt[]{x-1}+1)²}$
= $\sqrt[]{x-1}+1+\sqrt[]{x-1}-1$
= $2.\sqrt[]{x-1}$
