不知道解得对不对
程序：
dsolve('Dx=x+y','Dy=x-y','t')
解得：x=C1*exp(2^(1/2)*t)+C2*exp(-2^(1/2)*t)
y=C1*2^(1/2)*exp(2^(1/2)*t)-C2*2^(1/2)*exp(-2^(1/2)*t)-C1*exp(2^(1/2)*t)-C2*exp(-2^(1/2)*t)