数列｛an｝是首项为23，公差为整数的等差数列，且第六项为正，第七项为负. （1）求数列的公差； （2）求前n项和Sn的最大值； （3）当Sn＞0时，求n的
E Cafe Hub Near the Castle is The Hub, home of the Edinburgh International Festival and one of the most talked about spots in town. Our café is open every day for mouth-watering lunches, snacks and f
----How often do you _________ the dictionary when you are doing reading? ----Not very often. Only when I find it is necessary. A. relate to B. consult to C. refer to
已知x、y满足不等式组求使z=160x+252y取得最小值的非负整数点.
叶片与外界环境之间进行气体交换以及水分散失的“门户”是（ ）A．表皮 B．保卫细胞C．叶脉 D．气孔
内壁光滑的导热汽缸竖直浸人在盛有冰水混合物的水槽中，用不计质量的活塞封闭压强为1.0×l05pa，体积为2.0×10-3m3的理想气体，现在活塞上缓慢倒上沙
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则x的值为( ) 