Let a + b = c be the basic equation.

Then, both sides multiply with (a+b) .
(a + b)^2 = c (a+b) .
a^2 + 2ab +b^2 = ac + bc .

Move ab from left to right; and ac and b^2 from right to left.
a^2 + ab - ac =  bc - b^2 -ab .

Factoring …
a (a+b-c) =  -b (a + b -c)

Removing (a+b-c)
And we have:
a = -b.

Oops. It contradicts with our basic equation.

Obviously something is wrong here.

Guess what is the mistake here? It is a primary school’s question in fact 🙂