Not really the place to solve this, and this takes me back to my signal & systems days, but basically your equations should have C as a divider...
Remember it's
I = C *dV/dt
Therefore if you have dV/dt on the RHS, you should expect to see 1/C on the LHS:
function khan
t = 0:0.001:10;
x0 = 0;
R1 = 90000;
R2 = 1000;
C = 0.001;
options = odeset('RelTol',1e-6,'AbsTol',1e-8);
[t,x]=ode45(@rhs, t , x0,options);
plot(t,x);
function dxdt = rhs(t,x)
dxdt = -(1/R1 + 1/R2)*x/C + 1/R1*abs(sin(100*pi*t))/C ;
%It's form is dx/dt = -A.x + B.U(t)
end
end