MATLAB: MatLab wont let me use Ax as a variable

dsolve

Hi,
Im currently doing a university assignment about a packaged falling from a helicopter with drag force.
The problem I have is that matlab wont let me use the variable Ax, it lets me use A, AX, x, Axx, but when I use Ax i get an error message in my dsolve line. Whats going on??
%MOTION OF PACKAGE.
%Constant boundary conditions
syms Vx 
m=265; %Mass of package [kg]
g=9.81; %Acceleration due to gravity [ms^-2]
p=1.225; %Air density at sea level [kgm^-3].
%Boundary conditions parachute closed (0-3 seconds).
Axx=1; %Area facing horizontal air flow.
Cdx=1.3; %Horizontal drag coefficient.
%Create time row vectors for when parachure is open and closed.
t1=linspace(0,3,31); %Time 0-3 seconds (0.1 second steps).
%Equations to define motion of package (F=ma ODE).
eqn1 = '(0.5*p*Axx*Cdx)*Vx^2=m*DVx'; %x-axis velocity
%Solution for x-axis velocity when parachute is closed.
inits1 = 'Vx(0)=14.3053'; %Boundary condition, when t=0 seconds, Vx=32mph.
Vx1=dsolve(eqn1,inits1,'t1'); %Solve ODE using boundry conditions so that Vx1 is  a function of t1.
Vx1=eval(vectorize(Vx1)); %Evaluate solution Vx1 for all values of t1, create row vector of results.

Best Answer

syms Vx(t)
m = 265; %Mass of package [kg]
g = 9.81; %Acceleration due to gravity [ms^-2]
p = 1.225; %Air density at sea level [kgm^-3].
%Boundary conditions parachute closed (0-3 seconds).
Axx = 1; %Area facing horizontal air flow.
Cdx = 1.3; %Horizontal drag coefficient.
%Create time row vectors for when parachute is open and closed.
t1 = linspace(0,3,31); %Time 0-3 seconds (0.1 second steps).
DVx = diff(Vx);
%Equations to define motion of package (F=ma ODE).
eqn1 = (0.5*p*Axx*Cdx)*Vx^2==m*DVx; %x-axis velocity
%Solution for x-axis velocity when parachute is closed.
inits1 = Vx(0)==14.3053; %Boundary condition, when t=0 seconds, Vx=32mph.
Vx1sol = dsolve(eqn1, inits1); %Solve ODE using boundry conditions so that Vx1 is a function of t
Vx1 = double(subs(Vx1sol, t, t1));
plot(t1, Vx1)
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