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\(a,\left(\sqrt{2}+\sqrt{11}\right)^2=12+2\sqrt{22}\\ \left(\sqrt{3}+5\right)^2=28+10\sqrt{3}\)

Ta thấy \(12< 28;2\sqrt{22}=\sqrt{88}< \sqrt{300}=10\sqrt{3}\)

Nên \(\sqrt{2}+\sqrt{11}< \sqrt{3}+5\)

\(b,\left(\sqrt{21}-\sqrt{5}\right)^2=26-2\sqrt{105}\\ \left(\sqrt{20}-\sqrt{6}\right)^2=26-2\sqrt{120}\)

Vì \(\sqrt{105}< \sqrt{120}\Rightarrow-2\sqrt{105}>-2\sqrt{120}\)

Nên \(\sqrt{21}-\sqrt{5}>\sqrt{20}-\sqrt{6}\)

\(Q=\sqrt[3]{20+14\sqrt{2}}+\sqrt[3]{20-14\sqrt{2}}\)

\(=\sqrt[3]{8+12\sqrt{2}+12+2\sqrt{2}}+\sqrt[3]{8-12\sqrt{2}+12-2\sqrt{2}}\)

\(=\sqrt[3]{\left(2+\sqrt{2}\right)^3}+\sqrt[3]{\left(2-\sqrt{2}\right)^3}\)

\(=2+\sqrt{2}+2-\sqrt{2}=4\)

Làm tiếp nhé

\(\sqrt{3+\sqrt{20}}\)  và \(\sqrt{5+\sqrt{5}}\)

        2,7333...         và        2,6899...

TỪ ĐÓ TA THẤY ĐƯỢC RẰNG:

      2,7... > 2,6.... SUY RA  \(\sqrt{3+\sqrt{20}}\)>   \(\sqrt{5+\sqrt{5}}\)

\(\sqrt{3+\sqrt{20}}\)và \(\sqrt{5+\sqrt{5}}\)

Ta có:

      \(\sqrt{3+\sqrt{20}}\)

\(=\sqrt{3+\sqrt{2^2\times5}.}\)

\(=\sqrt{3+\sqrt{2^2}\sqrt{5}}.\)

\(=\sqrt{3+2\sqrt{5}}\)

\(=\sqrt{3}+\sqrt{2}\sqrt[4]{5}.\)

\(=2,73352...\)

      \(\sqrt{5+\sqrt{5}}\)

\(=\sqrt{5}+\sqrt[4]{5}.\)

\(=2,68999...\)

Suy ra:

\(2,73352...>2,68999...\)

Vậy:

\(\sqrt{3+\sqrt{20}}>\sqrt{5+\sqrt{5}}.\)

Học tốt nhé

\(\sqrt{12}<\sqrt{12,25}=3,5\)

\(\sqrt{20}<\sqrt{20,25}=4,5\)

\(\sqrt{30}<\sqrt{30,25}=5,5\)

\(\sqrt{42}<\sqrt{42,25}=6,5\)

Suy ra:\(\sqrt{12}+\sqrt{20}+\sqrt{30}+\sqrt{42}\)<3,5+4,5+5,5+6,5=20

Vậy biểu thức <20

\(\sqrt{12}+\sqrt{20}+\sqrt{30}+\sqrt{42}