---What's the matter with Tim?---Oh, Tim's cellphone was left in a taxi accidentally, never _____again.    

 

A. to find    B. to be found    C. finding    D. being found    

 

             

Work hard, and you will (success). Work hard, and you will (success).

测定铅锡合金中Pb、Sn含量时,称取试样0.2000g,用盐酸溶解后,准确加入50.00mL 0.03000mol/L的EDTA,50mL水,加热煮沸2min,

化简：. 化简：.

设Z1,Z2,Z3三点适合条件：Z1+Z2+Z3=0及|Z1|=|Z2|=|Z3|=1试证明Z1.Z2.Z3是内接于单位圆周|Z|=1的 正三角形的顶点.

设A是n阶矩阵,满足 ,则 _____. 设A是n阶矩阵,满足 ,则 _____.

设非负函数y=y(x)(x≥0)满足微分方程xy"-y'+2=0,当曲线=y(x)过原点时,其与直线x=1及y=0围成平面区域D的面积为2,求D绕y轴旋转所得旋

将298K,100kPa的 和398K,160kPa的He 都装到体积为 的容器中,并使体系温度保持在298K.计算混合后

计算：20＋20. 计算：20＋20.