等比数列中, 则的前项和为（ ） A. B. C. D.
About five years ago, when the first generation of wearable fitness trackers became popular, they were announced as the dawn of a revolution. Health experts and busniesspeople alike said tha
如图所示装置中，长木板甲重20N，物体乙重10N。甲、乙之间用一根轻绳通过定滑轮相连，当水平方向的拉力F的大小为10N时，长木板甲恰能在光滑的水平
There is an English saying, “Laughter is best medicine.” Until recently, few people took the saying seriously. Now, however, doctors have begun to look into laughter and the effects it has on th
用数学归纳法证明“5n-2n能被3整除”的第二步中,n=k+1时,为了使用假设,应将5k+1-2k+1变形为( )A.(5k-2k)+4×5k-2kB.5(5k-2k)+3×2kC.(5-2)(5k-2k)D.2(5k-2k)-3×5k
“鞍山轧钢开红花，‘解放’牌汽车跑中华，喷气式飞机翔蓝天，长江大桥武汉架。”这首歌谣所歌颂的成果的取得是在（ ） A．过渡时期
、陆地 B、海洋 C、所有的生物 D、生物圈