下列化学方程式错误的是( )
A:Fe2O3+3H2SO4 ==== Fe2(SO4)3+3H2O
B:Cu(OH)2+2HCl ==== CuCl2+2H2O
C:2Fe+6HCl ==== 3FeCl3+3H2↑
D:Ba(OH)2+ H2SO4 ==== BaSO4↓+2H2O
根据意思写词语。
没有经过邀请而突然到来的客人。____
He had hoped to have an opportunity to travel abroad for long. But it was a(n) ______ that he missed it when it came last month.
简谐运动是我们研究过的一种典型运动方式.
化简: x2−4x+4x2−4+x−2x2+2x+2 .