Question 4
(a) For integers $x,y$, not both zero, find the least value of $A=|5x^2+11xy-5y^2|$.
(b) Find all positive numbers $t$ such that $9t=10[t]/(t-[t])$, where $[t]$ denotes the integer part of $t$.
(b) Find all positive numbers $t$ such that $9t=10[t]/(t-[t])$, where $[t]$ denotes the integer part of $t$.