Let's simplify the given expression step by step:

cos (a - π) = cos (a - π)tan (π + a) = tan (π + a)sin (π/2 - a) = cos (a)

So, the expression simplifies to:

cos (a - π) tan (π + a) / sin (π/2 - a) = cos (a - π) tan (π + a) / sin (π/2 - a)

Since each part remains the same and cannot be further simplified, the final expression is:

cos (a - π) * tan (π + a) / sin (π/2 - a)