it is a maximization problem with no other constraint( a<b , n: integer , a,b>4 ,all variables positive )

I receive the same error when solving with optimization.( complex value or no-improved point)

then I tried to solve it by getting derivatives and using fsolve but again no answer!

I made it in a more simple format but again I cannot obtain any answer! the funcion is :

r := ((400*a*(1/a^0.1e-1))*exp(-.1*a)*(1-exp(-0.7e-1*t))/(0.7e-1)+

(400*b*(1/b^0.1e-1))*exp(-.1*b)*(1-exp(-0.7e-1*(10/n-t)))/(0.7e-1)-

.3*(400*exp(-.1*a)*(1-exp(-0.2e-1*t))/(a^0.1e-1*(0.2e-1))+400*exp(-.1*b)*(10*exp(0.5e-1*(10/n)^1.5)/n-t*exp(0.5e-1*t^1.5))/b^0.1e-1)-

.7*(-0.11666e-1*(10/n-t)^2*(.30*(10/n-t)^1.5*(8/n-.8*t-(2.8571*(1.5811*(1/n)^1.5+1))/n)-(30*(1.581*(1/n)^1.5+1))/n+20/n-2*t))-

90-

4*(400*exp(-.1*a)*(1-exp(-0.2e-1*t))/(a^0.1e-1*(0.2e-1))+400*exp(-.1*b)*(10*exp(0.5e-1*(10/n)^1.5)/n-t*exp(0.5e-1*t^1.5))/b^0.1e-1)-

.8*a*(400*t*exp(-.1*a)/a^0.1e-1-400*exp(-.1*a)*(1-exp(-0.2e-1*t))/(a^0.1e-1*(0.2e-1)))*exp(-0.7e-1*t)-

(1-exp(-0.7e-1*(10/n-t)))*(b-1)*((.3*400)*exp(-.1*b)/b^0.1e-1+.8*b)/(0.7e-1))  *  (1-exp(-0.7e-1*10)) / (1-exp(-0.7e-1*(10/n)))-90*exp(-0.7e-1*10)

it includes 4 positive variables( a,b,t,n) with constraint a<b