`a)`

`\sqrt{5^2-4^2}=\sqrt{25-16}=\sqrt{9}=3`

`b)`

`\sqrt{25^2-24^2}=\sqrt{(25-24)(25+24)}=\sqrt{49}=7`

`c)`

`\sqrt{85^2-84^2}=\sqrt{(85-84)(85+84)}=\sqrt{169}=13`

`d)`

`\sqrt{26^2-24^2}=\sqrt{(26-24)(26+24)}=\sqrt{50}=5\sqrt{2}`

_____________

`a)`

`\sqrt{x}=15 (đk:x≥0)`

`(\sqrt{x})^2=15^2`

`x=225 (TM)`

Vậy `x=225`

`b)`

`2\sqrt{x}=14 (đk:x≥0)`

`\sqrt{x}=7`

`(\sqrt{x})^2=7^2`

`x=49 (TM)`

Vậy `x=49`

`c)`

`\sqrt{x}<\sqrt{2} (đk:x≥0)`

`(\sqrt{x})^2<(\sqrt{2})^2`

`x<4`

Vậy `0≤x<4`

`d)`

`\sqrt{2x}<4 (đk:x≥0)`

`(\sqrt{2x})^2<4^2`

`2x<16`

`x<8`

Vậy `0≤x<8`