如图,AD是正五边形ABCDE的一条对角线,则∠BAD=( )
— Well, it's hard to say, probably great, or maybe not.
铜元素有质量数为63和65的两种天然同位素,它的近似相对原子质量是63.5,则同位素63Cu所占的质量百分比为( )
下图《合作之举》告诉我们( )