瞧瞧那个增得快 直线相关 http://www.dfanning.com/tips/exponent_2.png 指数相关 http://www.algebra.com/cgi-bin/plot-formula.mpl?expression=graph(400%2C300%2C-10%2C10%2C-10%2C10%2C2%5Ex)&x=0003