Step-by-step explanation:
a² = a × a
a > 1
a² = (1+d) × (1+d) = 1² + 2d + d²
with d > 0
since d > 0, then so is 2d and also d².
so, a² is the sum of 1² = 1 and something greater than 0.
so, a² > 1
