What is the function to use to blacken a region?

I have got into MATLAB just for a week or two, so I am having trouble with functions. Can anyone tell me what function I should use to darken the region bounded by a graph and x-axis?

[Dyuman Joshi]

patch

fill

area

[Lê]

clear
%% Plotting the graphs
% We have p(x) = 16 => p(x) - 16 = 0
f = @(x) (800000 .* exp(-x./5000)) ./ (x+20000) - 16;
fimplicit(f), grid on
xlabel('x'); ylabel('p(x)');

I tried to graph this using fimplicit but all I get is a blank screen, any idea why and what should I fix?

[Dyuman Joshi]

fimplicit() is used for a different purpose. You want to use fplot here -

%% Plotting the graphs
% We have p(x) = 16 => p(x) - 16 = 0
f = @(x) (800000 .* exp(-x./5000)) ./ (x+20000) - 16;
fplot(f, [-1 1]*5e3), grid on
xlabel('x'); ylabel('p(x)');

Answer 1

[Star Strider]

Perhaps this —

% clear
%% Plotting the graphs
% We have p(x) = 16 => p(x) - 16 = 0
f = @(x) (800000 .* exp(-x./5000)) ./ (x+20000);

xint = fzero(@(x)f(x)-16, 1)

xint = 3.7270e+03

x = linspace(0, 4000);

figure
plot(x, f(x), '-k', 'LineWidth',3)
hold on
patch([x flip(x)], [zeros(size(x))+16 flip(f(x))], [1 1 1]*0.8)
hold off
grid on
yline(16, '-k', 'LineWidth',2)
xlabel('x'); ylabel('p(x)');
text(xint, 16, sprintf('$(\\approx %.0f,16)$', xint), 'Interpreter','latex', 'Horiz','right', 'Vert','top', 'FontSize',14, 'FontWeight','bold')
pos = gca().Position;
xapf = @(x,pos,xl) pos(3)*(x-min(xl))/diff(xl)+pos(1);                % 'x' Annotation Position Function
yapf = @(y,pos,yl) pos(4)*(y-min(yl))/diff(yl)+pos(2);                % 'y' Annotation Position Function
annotation('arrow', xapf([1500 1000],pos,xlim), yapf([35 25],pos,ylim))
text(1500, 35, 'consumer surplus', 'Horiz','left', 'Vert','bottom', 'FontName','TimesRoman', 'FontSize',14)

Make appropriate changes to get the result you want.

.

[Lê]

I am just flabbergasted. Thank you so much.

[Star Strider]

My pleasure!

If my Answer helped you solve your problem, please Accept it!

.

[Lê]

Also, if I want to find the consumer surplus, just the function integral will do? By choosing x min = 0, and x max = xint.

[Star Strider]

Yes.

If you want the entire area from 0 to 4000, use those as the limits, otherwise use ‘xint’ as the upper limit —

f = @(x) (800000 .* exp(-x./5000)) ./ (x+20000);

format longG

xint = fzero(@(x)f(x)-16, 1)

xint = 
          3727.03867592715

Consumer_Surplus = integral(@(x)f(x)-16, 0, xint)

Consumer_Surplus = 
          37753.0101449318

Entire_Area = integral(@(x)f(x)-16, 0, 4000)

Entire_Area = 
          37611.8951504993

My apolgies for the delay — away doing other things for a few minutes.

.