solve equation system with dsolve

I am trying to solve a differential equation system with the command dsolve, but the answer is empty sym

I wrote this

syms x y

a = 0.1, b=0.2;

s = dsolve ('Dx=-a*x*y','Dy=-b*x*y','t')

an the system I want to solve is this

dx/dt=-axy

dy/dt=-bxy

thanks

[Star Strider]

It is a system of nonlinear differential equations. Only a few nonlinear differential equations have analytic solutions. These are not among them.