设(1＋3i)z=bi,则z=(bi)/(1＋3i),w=z/(2＋i)=(bi)/[(1＋3i)(2＋i)],|w|=|(bi)/[(1＋3i)(2＋i)]|=|b|/[√10×√5]=5√2,解得|b|=50,则：w=(bi)/[(1＋3i)(2＋i)]=±(1－i)