Smart problem

1) -20=-20

2) 16-36=25-45

3) 16-36+ $\frac{81}{4}$ =25-45+ $\frac{81}{4}$

4) 4^{2}+( $\frac{9}{2}$ )^{2}-2x4x $\frac{9}{2}$ =5^{2}+( $\frac{9}{2}$ )^{2}-2x5x $\frac{9}{2}$

5) (4- $\frac{9}{2}$ )^{2}=(5- $\frac{9}{2}$ )^{2}

6) ((4- $\frac{9}{2}$ )^{2})^{0.5}=((5- $\frac{9}{2}$ )^{2})^{0.5}

7) 4- $\frac{9}{2}$ =5- $\frac{9}{2}$

8) 4=5

9) 2+2=5

Where is the first mistake occurs?