The best way of learning a language is using it. The best way of learning English is talking in English as much as possible. Sometimes you'll get your words mixed up(混合) and people will not understand you. Sometimes people will say things too quickly and you can' t understand them. But if you keep your sense of humor(幽默感)，you can always have a good laugh at the mistakes you make. Don't be unhappy if people seem to be laughing at your mistakes. It's better for people to laugh at your mistakes than to be angry with you，because they don't understand what you are saying. The most important thing for learning English is："Don't be afraid of making mistakes because everyone makes mistakes."

若a＞b，则下列不等式变形错误的是（ ）

如图，矩形ABCD的对角形AC，BD交于点$O$，若$\angle BOC={120}^{°}$，$AB=4.8cm$，则对角线$BD$的长等于