设函数f(x,y)为可微函数，且对任意的x,y都有则使不等式成立的一个充分条件是
________somanypeopleintheUnitedStatesbeenoutofworkastoday.
Shehadclearlyno________ofdoinganywork,althoughshewasverywellpaid.
heteachertoldthemsincelight________(travel)fasterthansound,lightning________(appear)togobeforethunder.
Wemust______thattheexperimentiscontrolledasrigidlyaspossible.
上具有连续导数，则()