principle of least heat:
H=I12 R1t + (I-I1)2R2t
......................................................................> DISTRIBUTING CURRENT IN PARALLEL CIRCUIT WHERE TOTAL CURRENT FROM EXT SOURCE =I
d/dt (H) = 0..........at a fixed time (t) total heat generated in the circuit is minimum.
differentiating the above eqn. and putting value of differential = 0 , we get
I1R1 = I2R2