题目

先化简，再求值：－2x2－2[3y2－2(x2－ y2)＋6]，其中x＝－1，y＝－2． 答案：－2x2－2[3y2－2(x2－y2)＋6] =－2x2－2[3y2－2x2  + 2y2＋6]································································· 3分 =－2x2－6y2 + 4x2 －4y2－12···································································· 4分 =2x2－10y2 －12 ··�如图所示，物体A放在物体B上，物体B放在光滑的水平面上，已知mA=6kg，mB=2kg，A、B间动摩擦因数μ＝0.2，A物上系一细线，细线能承受的最大拉力是20N，水平向右拉细线，假设最大静摩擦力等于滑动摩擦力。下述中正确的是(g＝10m/s2)     A．当拉力F<12N时，A静止不动 B．当拉力F>12N时，A相对B滑动 C．当拉力F＝16N时，B受A摩擦力等于4N D．只要绳子未被拉断，A相对B始终静止