# How to shade the area bounded by curves

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Joemama22 on 2 Nov 2015
Commented: Star Strider on 20 Nov 2022
I need to shade the area bounded by the curves y=x^2, the x-axis, and y= -(1/4)*x+(9/2) the color yellow. I can't figure out how to do this. I'm close, and I'm sure it's an easy fix, but here's what I have so far:
clc;clear;close all;
syms x;
par= int(x^2,0,2);
dy= diff(x^2);
lin= int(-(1/4)*x+(9/2),2,18);
y1=x^2;
y2=-(1/4)*x+(9/2);
area=(par+lin)
hold on
fplot('x^2',[0 2])
fplot('-(1/4)*x+(9/2)',[2 18])
%fill(x,y,'y')
hold off
title('Supplement 7: Problem 4')
axis equal; axis([0 20 0 5])
set(gca, 'Xtick', 0:20)
set(gca, 'Ytick', 0:5)
xlabel('x'); ylabel('y')

Star Strider on 2 Nov 2015
A somewhat simpler approach:
x=linspace(0, 18);
y=x.^2;
y2=x.*(-1./4)+(9./2);
y3=x.*0;
figure(1)
plot(x,y, x,y2, x,y3); grid on;
axis([-1,20,-1,5])
area(x, min([y; y2]), 'FaceColor','y')
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Star Strider on 20 Nov 2022
@Joshua — I have no idea what ‘isn’t working’ means.
That consideration aside, patch is an option —
x=linspace(0, 18);
y=x.^2;
y2=x.*(-1./4)+(9./2);
y3=x.*0;
figure(1)
plot(x,y, x,y2, x,y3); grid on;
axis([-1,20,-1,5])
hold on
patch([x flip(x)], [min([y; y2]) zeros(size(x))], 'y', 'EdgeColor','none')
hold off
The code changes slightly with patch in order to create a closed region for it to fill.
.

TastyPastry on 2 Nov 2015
This should do it. Modified from this.
t=(-10:0.01:10)'; %values on x axes
f=t.^2; %values on y axes, put your function here
mif=zeros(numel(t),1);
maf=-.25.*t+4.5;
%select minimum and maximum value (y axes)
cla
axis([-10 10 -10 10])
hold on
line(xlim,[mif mif],'LineStyle','Color','k')
line([t(1) t(end)],[maf(1) maf(end)],'LineStyle','--','Color','g')
idx=f>mif & f<maf; %select the index values of f that satisfy the conditions
plot(t,f) %plot the function
fill(t(idx),f(idx),'r') %fill the area that's inside the conditions
legend('maximum','minimum','function','area')
Joemama22 on 2 Nov 2015
When I put this code into matlab, nothing happens.