已知质量数为A的某阳离子Rn+,核外有x个电子,则核内中子数为( )
A:A-X
B:A-X-n
C:A-X+n
D:A+X-n
如图,AB∥CD,∠CDE=140°,则∠A的度数为( )
—I can’t find the answer to the question .
—Think hard, and you can_______.
若实数a,b 满足1a+2b=ab,则ab的最小值为( )