$\\$
`a,`
`4 \sqrt{x}-17 = \sqrt{25}`
`-> 4\sqrt{x}-17=5`
`-> 4 \sqrt{x}=5+17`
`-> 4\sqrt{x}=22`
`-> \sqrt{x}=22:4`
`-> \sqrt{x}=11/2`
`-> \sqrt{x}=\sqrt{121/4}`
`->x=121/4`
Vậy `x=121/4`
$\\$
`b,`
`3x^2 +15=42`
`-> 3x^2=42-15`
`->3x^2=27`
`->x^2=27 : 3`
`->x^2=9`
TH1 :
`->x^2=3^2`
`->x=3`
TH2 :
`-> x^2=(-3)^2`
`->x=-3`
Vậy `x=3` hoặc `x=-3`
$\\$
`c,`
`3 (x^2-1) =\sqrt{49}`
`->3 (x^2-1)=7`
`-> x^2-1=7:3`
`->x^2-1=7/3`
`->x^2=7/3+1`
`->x^2=10/3`
TH1 :
`->x^2=(\sqrt{10}/3)^2`
`->x=\sqrt{10/3}`
TH2 :
`-> x^2=(-\sqrt{10/3})^2`
`->x=-\sqrt{10/3}`
Vậy `x=\sqrt{10/3}` hoặc `x=-\sqrt{10/3}`
$\$
`d,`
`(2\sqrt{x}+12)/3=\sqrt{100}`
`-> (2\sqrt{x}+12)/3=10`
`-> 2\sqrt{x}+12=3.10`
`-> 2\sqrt{x}+12=30`
`->2 \sqrt{x}=30-12`
`->2 \sqrt{x}=18`
`-> \sqrt{x}=18:2`
`->\sqrt{x}=9`
`-> \sqrt{x}=\sqrt{81}`
`->x=81`
Vậy `x=81`
