f(x)=-(x-1)/(2x+5)
=-(x+2.5-3.5)/2(x+2.5)
=-[(x+2.5)/2(x+2.5)-3.5/2(x+2.5)]
=-1/2+3.5/(2x+5)
2x+5是增函数
则2x+5>0和2x-5