Question 834668
{{{(x^4)-5(x^2)(y^2)+(4y^4)}}}
Let a = x^2
    b = y^2
and the equation becomes
{{{(a^2)-5(a)(b)+(4b^2)}}}
or
{{{a^2-5ab+4b^2}}}
now factor
{{{(a - 1)(a - 4)}}}
substituting back
{{{(x^2 - 1)(y^2 - 4)}}}
Now we need to factor further
{{{(x + 1)(x - 1)(y + 2)( y - 2 )}}}