Noticed something weird in derivation of relationship between ICBO and ICEO in BJTs
The original equations are:
$$I_C = \alpha I_E + I_{CBO} \tag{a}$$
$$I_C = \beta I_B + (\beta +1)I_{CBO} \tag{b}$$

Before equation 4, you mentioned that \$\alpha I_E = I_C\$. But this is valid only when \$I_{CBO} = 0\$ and that is what equation 4 says. 

In the same way, \$I_C = \beta I_B\$ is valid only if \$I_{CBO} = 0\$ and that is what equation 5 says.

Both equation 4 and 5 are valid only under the approximation that \$I_{CBO} = 0\$.