`9)`
`a)\sqrt{x^2}=7`
`<=>|x|=7`
`<=>x=+-7`
~~~~~~~~~~~~~
`b)\sqrt{x^2}=|-8|`
`<=>|x|=8`
`<=>x=+-8`
~~~~~~~~~~~
`c)\sqrt{4x^2}=6`
`<=>|2x|=6`
`<=>2x=+-6`
`<=>x=+-3`
~~~~~~~~~~~
`d)\sqrt{9x^2}=|-12|`
`<=>|3x|=12`
`<=>3x=+-12`
`<=>x=+-4`
____________________________________
`13)`
`a)2\sqrt{a^2}-5a` `(a < 0)`
`=2|a|-5a`
`=-2a-5a=-7a`
~~~~~~~~~~~~~~~~
`b)\sqrt{25a^2}+3a` `(a >= 0)`
`=|5a|+3a`
`=5a+3a=8a`
~~~~~~~~~~~~~~~~
`c)\sqrt{9a^4}+3a^2`
`=|3a^2|+3a^2`
`=3a^2+3a^2=6a^2`
~~~~~~~~~~~~~~
`d)5\sqrt{4a^6}-3a^3` `(a < 0)`
`=5|4a^3|-3a^3`
`=-20a^3-3a^3=-23a^3`
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