Square and Square Roots

The difference of two perfect squares is a perfect square.
If true then enter $$1$$ and if false then enter $$0$$
One of the following number is not a perfect square, state the number.  
$$2453, 2500$$
If $$2l - 3m = -1$$ and $$lm = 20$$, then the value of $$4l^{2} + 9m^{2}$$ is ________.
If a, b and c are positive integers, the radicals $$\sqrt{a + \dfrac{b}{c}}$$ and $$a\sqrt{\dfrac{b}{c}}$$ are equal when and only when: