Salman Khan's educational website of 2,400 video lessons could be the solution to middle-of-the-class mediocrity. Matthew Carpenter, age 10, has completed 642 inverse trigonometry problems at KhanAcademy.org. Photo: Joe Pugliese "This," says Matthew Carpenter, "is my favorite exercise." I peer over his shoulder at his laptop screen to see the math problem the fifth grader is pondering. It's an inv
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