多项式3x2-2x+1与-x2+2x+1的差等于______.
3x2-2x+1-(-x2+2x+1)=3x2-2x+1+x2-2x-1=4x2-4x.
故答案是:4x2-4x.
You may use the book as you like, _______ you keep it clean.
A.unless
B.so long
C.as long
D.so long as
机械加压送风系统主要由送风口、送风管、送风机和吸风口组成,其中非金属管道的风速不应大于()m/s。
A.5
B.10
C.15
D.20