证型衫明:因为f(x)和g(x)在区间D上都是增函数,所以f(x2)-f(x1)/(x2-x1)>埋腔0g(x2)-g(x1)/(x2-x1)>0,h(x2)-h(x1)=f(x2)+g(x2)-f(x1)-g(x1)/(x2-x1)=f(x2)-f(x1)/(x2-x1)+g(x2)-g(x1)/(x2-x1)>0所以函数h(x)=f(x)+g(x)在区间卜液腔D上也是增函数。(