# Finding the Area and Perimeter

A way to find the region such as the area is using $Integrate[]$, where you take the upper - lower or $f[x]-g[x]$, while the perimeter we add the upper and lower region after we found its arc length. However, I'm having trouble finding the area and perimeter using programming.

Given a function $areaPerimeter[f,g,x]$, I am trying to find if the curves intersect ie. $y=f(x)$ and $y=g(x)$.

Using $areaPerimeter[1/10 x^4 + x + 1, x^2 - 1, x]$, I need to display the plot and PlotLabels showing the ${area,perimeter}$ above the plot

My attempt:

areaPerimeter[f_, g_] :=
Plot[{f[x], g[x]}, {x, -2, 4}]


Then I indicated the parameters for the overall plot using Module

areaPerimeter[f_, g_, x_] := Module[{sols, xvals, min, max, p, l},
sols = NSolve[f == g, x, Reals];
xvals = x /. sols;
min = Min[xvals];
max = Max[xvals];
d = max - min;
If[Length[sols] == 0,
min = -5;
max = 5;
d = 0
];
If[Length[sols] == 1,
d = 10;
];
p = Plot[{f, g}, {x, min - 0.2*d, max + 0.2*d}];
l = ListPlot[{x, f} /. sols, PlotStyle -> Black];
Show[p, l]
]


pts = {x, x^2 - 1} /.
NSolve[{x^2 - 1 == 1/10 x^4 + x + 1}, x, Reals, WorkingPrecision -> 15];

reg = ImplicitRegion[
1/10 x^4 + x + 1 <= y <= x^2 - 1 &&
pts[[1, 1]] <= x <= pts[[2, 1]], {x, y}];

area = Area[reg]

(* 4.19337506922227 *)


Verifying,

area == NIntegrate[1, {x, pts[[1, 1]], pts[[2, 1]]},
{y, 1/10 x^4 + x + 1, x^2 - 1}]

(* True *)

perimeter = RegionMeasure@RegionBoundary[reg]

(* 21.4130315179935 *)


Verifying,

perimeter ==
Total[ArcLength[{x, #}, {x, pts[[1, 1]], pts[[2, 1]]},
WorkingPrecision -> 15] & /@ {x^2 - 1, 1/10 x^4 + x + 1}]

(* True *)

Plot[{x^2 - 1, 1/10 x^4 + x + 1}, {x, -3.5, -0.75},
Filling -> {1 -> {{2}, {White, LightBlue}}},
Epilog -> {Text[
"area = " <> ToString@NumberForm[area, {6, 3}], {-2, 10}, {1, 1}],
Text["perimeter = " <> ToString@NumberForm[perimeter, {6, 3}], {-2,
11}, {1, 1}],
Red, AbsolutePointSize, Point[pts]},
PlotLegends -> Placed["Expressions", {0.6, 0.5}]] Plot[{x^4/10 + x + 1, x^2 - 1}, {x, -4, 0}, PlotLegends -> Automatic] reg = ImplicitRegion[x^4/10 + x + 1 <= y <= x^2 - 1, {x, y}];
Area@reg // N
4.19338

Perimeter@reg // N
21.413


Perimeterintroduced in 2017 (11.1)

• What's with that clean up you've done with your answers? :) – Kuba Nov 13 '18 at 21:14