Math.

Let a=b
aa = ab

-bb to both sides...
aa - bb = ab - bb

Factorise,

(a+b)(a-b) = b(a-b)

Cacellation of common factors,

a+b = b

Sub a=b.

2a = a

Divide both sides by a,

2 = 1

Find a and b.

Courtesy of the Bio relief teacher my class had...