だから
S=S(C1)∪S(C2)-S(C1)∩S(C2)
=S(C1)+S(C2)-2S(C1)∩S(C2)
=π(1/2)^2+π(1/√2)^2-2(π/8+π/8-1/4)
=π/4+1/2
でええやん!!