Maximum Power Transfer to R Load
V  1 0 12V
R1 1 2 20
R2 2 0 20
R3 2 3 20
RL 3 0 RLOAD 1
.MODEL RLOAD RES
.DC RES RLOAD (R) 10 40 2
.option nopage
.PRINT DC V(RL) V(R1) 
.PLOT DC V(RL)  V(R1)
.PROBE
.OP
.END