18.解:(1)根据题意可知,带电粒子的运动轨迹如图所示在第三象限,根据动能定理有gU=m2(2分)C解得;=(1分)M(2)设粒子在第二象限内的运动时间为t在y轴方向有L=t(2分)1.E在x轴方向有号L=2··2(2分)联立解得公3U(1分)(3)在第一象限内,设粒子离开第二象限时沿x轴正方向的分速度大小为v,合速度大小为以,速度v与y轴正方向的夹角为0则L=:(2分)tan==3(1分)粒子在磁场中运动的速度v=(1分COs由洛伦兹力提供向心力有B=m三(1分)矩形磁场的最小面积是图中以QA为长,AC为宽的矩形面积QA=2rsin=L(1分)AC=r-rcos 8=L(1分)6解得S=Q4XAC=L2。(1分)

第四部分写作第一节短文改错I like playing computer games and ny/ dream is 1oecome a computer programmer. I think computerprogrammers are A great demand in the modern worldBesides, my parents would like me to study medicineoweverThey believe there are two reason for being a doctor. Onereasonsreason is that doctors usually look after patientspersonally, means they are able to help people in themeaningneed directlv. The other is that doctors are very importantthat it is almost impossible for it to lose jobs. Up tothemnow I had chatted with my parents several times. Butavegreat to my disappointment. I failed to persuade themgreatlyNow I am confusing about how to solve the problemd

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