If the constant term in the expansion of $${\left( {\sqrt x - {k \over {{x^2}}}} \right)^{10}}$$ is 405, then what can be the values of k?

What is the number of terms in the expansion of $${[{(2x - 3y)^2}{(2x + 3y)^2}]^2}$$ ?

In the expansion of (1 + ax)ⁿ, the first three terms are respectively 1, 12x and 64x². What is n equal to?

 If $x=(1111){}^2$, $y=(1001){}^2$ and $z=(110){}^2$, then what is x³−y³−z³−3xyz equal to?