c=3

c

(a^2)+3a+1/.{(a^2)->2}

a

a=3b

(a^2)

a

c={{1,0},{0,1}}
d={{1,0},{3,2}}
c.d//MatrixForm

MatrixForm[c]

b={3,1,{5,{2,3}}}

b[[3,2,2]]

Solve[{(y^2)-1==x,(x^2)+y==1},{x,y}]

sol=DSolve[m x''[t]+(m/τ)x'[t]-e El==0,x[t],t]

sol[[1]]

D[(t^2),t]

D[x[t]/.sol[[1]],t]

<<PhysicalConstants`
<<Units`

(* Por ejemplo para GaAs tenemos *)
uμe=80 ((Meter^2)/(Volt Second))
uτ=Convert[((ElectronMass 0.067 uμe)/ElectronCharge) ,Pico Second]

uL = 1 Milli Meter;
uV= 5 Volt;
uEl=(uV/uL)
uv=Convert[uEl uμe,(Meter/Second)]
(* de un extremo de la muestra al otro se tarda ...*)
Convert[(uL/uv) ,Pico Second]

(* Difusion *)
T=4;
uDe=Convert[((uμe BoltzmannConstant T)/ElectronCharge),((Meter^2)/Second)]

L0=30.0 Micro Meter;
V0=1.0 Volt
t0=10 Pico Second;
T=40 Kelvin;
L=Convert[uL/L0,1]
V=Convert[uV/V0,1]
μe = Convert[((uμe V0 t0)/(L0^2)),1]
μh =μe/2 
De=Convert[((uDe t0)/(L0^2)),1]
Dh=De/2
El=Convert[(uEl/V0)L0,1]

solp=DSolve[{μh p'[x]El-Dh p''[x]==0,p[0]==20,p[L]==400},p[x],x]
soln=DSolve[{-μe n'[x]El-De n''[x]==0,n[0]==20,n[L]==},n[x],x]

n0[x_]=((n[x]/.soln[[1]])/.{C[1]->N1,C[2]->N2})
p0[x_]=((p[x]/.solp[[1]])/.{C[1]->P1,C[2]->P2})