How do I prove that SAT in coNP implies NP=coNP?
If SAT is in coNP then since coSAT is coNP-complete, SAT can be reduced to coSAT in polytime. The same reduction also reduces coSAT to SAT. Since coSAT is coNP-complete, every problem in coNP can be reduced in polytime to coSAT, and so to SAT. This shows that SAT is coNP-complete.