I just saw this equation and I can't find out where's the problem:
$$25-45 = 16-36$$
$$25- 2 \cdot 5 \cdot \frac{9}{2} = 16- 2\cdot4\cdot\frac{9}{2}$$
$$25 - 2\cdot 5\cdot \frac{9}{2} + \frac{81}{4} = 16 - 2\cdot 4 \cdot \frac{9}{2} + \frac{81}{4}$$
$$\left( 5-\frac{9}{2} \right) ^2 = \left (4-\frac{9}{2} \right) ^2$$
$$5-\frac{9}{2} = 4 - \frac{9}{2}$$ $$5=4$$