lst = {
{100, 0}, {110, 25}, {120, 30}, {130, 50}, {140, 70},
{150, 60}, {160, 50}, {170, 40}, {160, 30}, {150, 20}, {140,
10}, {130, 0}
};
lngth = Length[lst];
ListPlot[lst, AxesOrigin -> {0, 0}]
Range[lngth];
Transpose[{%, lst}]
lstInt = Interpolation[%]
Plot[lstInt[x][[1]], {x, 1, lngth}]
Plot[lstInt[x][[2]], {x, 1, lngth}]
Show[
ListPlot[lstInt[#] & /@ Range[lngth], AxesOrigin -> {0, 0},
PlotStyle -> {Green}],
ListPlot[lstInt[#] & /@ (Range[lngth - 1] + 0.2),
AxesOrigin -> {0, 0}, PlotStyle -> {Red}],
ParametricPlot[{lstInt[x][[1]], lstInt[x][[2]]}, {x, 1, lngth},
AspectRatio -> 0.5]
]
FindRoot[lstInt[x][[2]] == 20, {x, lngth}]
lstInt[x] /. %