At least one of the numbers is less than 1.

A*b = a + b - 1

a*b - a = b - 1 (subtract a from both sides)

a(b-1) = b-1 (factor out a)

a = (b-1)/(b-1) (divide by b-1)

a = 1

The problem is symmetrical, so

1*1 = 1 + 1 - 1

It is interesting that (b-1)/(b-1) becomes an indeterminate form when b=1, yet the answer still holds.

A*b = a + b - 1

a*b - a = b - 1 (subtract a from both sides)

a(b-1) = b-1 (factor out a)

a = (b-1)/(b-1) (divide by b-1)

a = 1

The problem is symmetrical, so

**both numbers must be 1**.1*1 = 1 + 1 - 1

It is interesting that (b-1)/(b-1) becomes an indeterminate form when b=1, yet the answer still holds.