桥式起重机“制动器制动不灵刹不住车，起升机构溜钩，运行机构溜车，断电后制动滑行距离过大”的现象，引起的可能原因有（）。
A、制动瓦衬胶粘在制动轮上BCD
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已知，二次型的秩为2，(I)求实数a的值；(II)求正交变换x=Qy将f化为标准形.
Matteristhename________(give)toeverythingwhichhasweightandoccupiesspace.
EachyearintheUnitedStates,manyblackteenagers_______ofschool,eitherbecausetheycannotkeepuportheyhavetoworktosupporttheirfamily.
设曲线y=ax^2+2x在点(1,a+2)处的切线与直线y=4x平行，则a=（）
Wooddoesnotconductelectricity;________.
