7.a>1且b>1”是logb>0(a>0且a≠1)”的A.充分不必要条件B.必要不充分条件C.充要条件D.既不充分也不必要条件2x-y+4≥0,8.已知实数x,y满足约束条件x-y-1≤0,则目标函数z=y-2x的最小值为x+2y-1≤0,C.-1D.-2A.2B.1已知数列(a)满足a2,4aga,(是等差数列,则数列(-1a)的前10S10=A.220B.110C.99D.5510.n个正数的和与这n个正数的倒数和的乘积的最小值是D1A.1 B. n C.n