∴Dafydd is correct.
Explanation:
Dafydd chooses three integers a, b and c .
b is 5 less than a and c is 5 more than a.
Dafydd says that bc +25 = a²
Here b = a-5 and c= a+5
L.H.S = bc +25
= (a-5)(a+5) +25
= a² - 5² + 25
=a² -25 +25
= a² = R.H.S
∴Dafydd is correct.