>>215
(a,b)∈(R1∪R2)^-1⇔(b,a)∈R1∪R2⇔(b,a)∈R1∨(b,a)∈R2
⇔(a,b)∈R1^-1∨(a,b)∈R2^-1⇔(a,b)∈R1^-1∪R2^-1