Question

what is a perfect square number i know 9, 25 and 100, but what else

if you multiply a perfect square by a perfect square, is the answer always a perfect square

Answer 1

Yes, this might be easier to understand if you think of expressing the perfect square as a product of its square roots. Rearrange to see that the product is a perfect square

if x and y are perfect squares:
x = a * a
y = b * b
x * y = a * a * b * b
x * y = (a * b) * (a * b)
let c = a * b
x * y = c * c