$(20$pts$)$ Compute the weakest precondition for each of the following assignment statements and postconditions.

Detailed steps are needed.

a$)a=2**(b-1)-1\{a>0\}$

b$)a=2**b+1$

$b=a-3\{b<mn>0</mn><mi>\}</mi>$

c$)$ if $(a==b$

$b=2**a+1$

else

$b=2**a$

$\{b>1\}$