The length of a rectangle is a centimeter, and the width is b centimeters. If the length and width are increased by 1 centimeter, then the area is increased by a+b+ how many square centimeters.
一个长方形的长是a厘米，宽是b厘米，如果将长、宽都增加1厘米，那么面积增加a+b+多少平方厘米．
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