用定义法：根据定义域 (－∞,－1) ,令x1＜x2＜-1f(x2)-f(x1) = (x2²+2x2) - (x1²+2x1) = (x2²-x1²) + (2x2-2x1) = (x2+x1)(x2-x1)+2(x2-x1) = (x2-x1)(x2+x1+2)∵x1＜x2＜-1∴(x2-x1)＞0,(x2+x1...