胸内负压形成的主要原因是
A
暂无解析
________(fail)severaltimes,theyneedsomeencouragement.
Thefloordoesnotlooksobadwhen________(sweep)clean.
Iknowitisn’timportantbutIcan’thelp________aboutit.
f(x,y)=y根号1-x2+sf(x,y)dxdy其中D=（x,y），则
设A为三阶矩阵,P为可逆矩阵,使得P-1AP=012