Question 1
(a) For which positive integers $n$ is $\underbrace{11\ldots1}_{2n\text{ digits}}-\underbrace{77\ldots7}_{n\text{ digits}}$ a perfect square?
(b) Determine all digits $b$ for which $\underbrace{11\ldots1}_{2n\text{ digits}}-\underbrace{bb\ldots b}_{n\text{ digits}}$ is a perfect square.
(b) Determine all digits $b$ for which $\underbrace{11\ldots1}_{2n\text{ digits}}-\underbrace{bb\ldots b}_{n\text{ digits}}$ is a perfect square.