HP 48SX real/complex algebra

[Gil]

The calculator cannot guess that A, B, C and D are supposed to be real.

Put RE(A), instead of A
Put RE(B), instead of B
Put RE(C), instead of C
Put RE(D), instead of D

'RE((RE(A)+RE(B)*i)*(RE(C)+RE(D)*i))'


And you will get the simplified "expected" answer with each time RE before the letters A, B, C and D:

'-(RE(D)*RE(B)-RE(C)*RE(A))'