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.
设(I)计算行列式A；(II)当实数a为何值时，方程组有无穷多解，并求其通解
SinceWilliamhadbeenseriouslyillforseveralmonths,hisparentswereworriedabout________toreturntoschoolfull-time.
Thewealthofacountryshouldbemeasured________thehealthandhappinessofitspeopleaswellasthematerialgoodsitcanproduce.
设f(x,y)具有一阶偏导数，且对任意的(x,y)，
，其中n为正整数,则f'(0)=()