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如题l2图，磁感应强度为B的均匀磁场水平向右，长度为l的直导线ab于位子纸面内，与磁场方向间夹角为θ，在磁场中以速度v垂直纸面运动。则直导线ab中电动势的大小为
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4. Using medicines derived from plants is a practice probably as old as humankind itself. Prehistoric peoples likely noted when consuming a particular plant part provided relief, such as willow bark "tea" lowering a fever. Sumatran (苏门答腊的) clay tablets engraved forty centuries ago list plant-based remedies for common ills, as do ancient writings from Egypt and China. In nineteenth-century United States, St. John's wort Echinacea were just two of many commonly used herbal remedies.Many modern medicines are synthetic versions of plant-derived "natural products." A compound from a periwinkle plant, for example, served as the basis for a powerful drug that fights leukemia (白血病). Poppies provide alkaloids such as morphine that are potent painkillers.In the U.S. today, one-third of all adults have tried herbal treatments, creating a multibillion-dollar market. The resurgence of interest in herbal medicine is largely due to the Dietary Supplements Health Education Act (DSHEA. of 1994, which expanded the definition of "dietary supplement" beyond essential nutrients to include "herbs botanicals," thus removing them from regulation as drugs. This designation means that labels can only mention ways that the herbal product can promote health, not cure disease. For example, valerian root "promotes restful sleep," St. John's wort "may help enhance mood," and Echinacea goldenseal "may help support the immune system." Many physicians biochemists argue that active ingredients in many herbal remedies are indeed drugs, should be regulated as such.It is wise to consult a physician when considering use of an herbal product. Even for a well-understood remedy such as cranberry extract, additional therapy may be required, or drug interactions a possibility. The law may not currently consider herbal ingredients to be drugs, but science indicates otherwise.
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Excel中，输入公式“=SUM(A1:B3)”，则求和的单元格的个数为______。
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