x1 = 2.8
y1 = 34
x2 = 3.1
y2 = 39
当x = 2.9
y = (x - x1)(y2 - y1) / (x2 - x1)+ y1
= (2.9 - 2.8)(39 - 34) / (3.1 - 2.8) + 34
= 35.667
细度模式2.9用内插法求砂率率是 35.667