Two wires of resistances R1 and R 2 have temperature coeffcient of resistances $ \alpha _1 $ and $ \alpha_2 $ respectively they are joined in series the effective temperature coefficient of resistance is ....
$ R_it = R_1 ( 1 + \alpha it ) R_2 = R_2 ( 1 + \alpha_2 t )$ $ R_st = R_1 t + R_2 t $ $ = R_1[ 1 + \alpha_1 t ] + R_2[ 1 + \alpha_2 t ] $ $ = R_s [ 1+ ({(R_1 \alpha_1 +R_1 \alpha_2 t )\over R_1+R_2})]$