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More than 2 million US teenagers have suffered a serious bout（发作) of depression in the past year, according to a federal government survey released on Tuesday. On average, 8.5 percent of adolescents aged 12 to 17 described having had a major depressive episode in the previous year, the Substance Abuse and Mental Health Services Administration reported. But there were striking differences by sex, with 12.7 percent of girls and 4.6 percent of boys affected. Depression is the leading cause of suicide, which in turn is the third-leading cause of death for 15- to 24-year-olds in the United St4．当x＜1时，分式$\frac{1}{1-x}$有值为正数．