I have tried using 2 method but I'm getting different answers in both methods.

in the first case I assumed Vce=Vcc/2 and Vcc-IcRc=Vce

For maximum power transfer Vcc/(2√2)×Icc/(2√2)=Vcc×Icc/8=20W

Vcc/2=IcRc

RC comes out to be 1.8

But when I go for the formula (Vcc)^2/(8Rc)=20W I'm getting Rc=3.6 why is that so??