因π/2<2α<π,-π/4<-β<0,故π/4<2α-β<π，又因为cos（2α-β）=-二分之根号二，所以2α-β=3π/4因π/4<α<π/2,-π/2<-2β<0,故-π/4<α-2β<π/2，又因为sin(α-2β）=二分之根号二，所以α-2β=π/4所以a+β=（2α-β）-（α-2β）=π/2所以cos(a+β）=cosπ/2=1
