Inequality

[man111]

If $xyzt=81,x,y,z,t>0$ Then minimum of $x+2y+z-8t$ is

After editing

[wrest]

$t=80,(9); -8t=-647,(9)$

[kotenok gav]

-620.

-- 15 сен 2017, 02:08 --

$t<81$

-- 15 сен 2017, 02:09 --

$80.(9)=81$

[man111]

Thanks friends actually it is $xyzt=81$

[wrest]

$- \infty$ if $x,y,z,t$ are positive reals
$-644$ if $x,y,z,t$ are positive integers
where's a trick?