%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% doubletrapstep.m
% Double trapstep.m
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

function y = doubletrapstep(t,x,h, friction)
%one step of the trapezoid method
z1=doubleydot(t,x, friction);
g=x+h*z1;
z2=doubleydot(t+h,g, friction);
y=x+h*(z1+z2)/2;