factorise:$5"x"^2 - (20"x"^4)/9$
$5"x"^2 - (20"x"^4)/9$
$= 5"x"^2[1 - "4x"^2/9]$
$= 5"x"^2 [1 - "2x"/3][1 + "2x"/3]$
