This is mostly explained in the previous exercise.

When it comes to par1 and par2 specifically, we should examine
the formulas themselves:
((R1+-t1*R1)*(R2+-t2*R2)) / (R1+-t1*R1 + R2+-t2*R2) =
(R1*R2 +-t1*R1*R2 +-t2*R2*R1 +-t1*t2*R1*R2) /
    (R1+R2+-t1*R1+-t2*R2) =
(R1*R2 +-t1*R1*R2 +-t2*R2*R1 +-t1*t2*R1*R2) /
    (R1+R2+-t1*R1+-t2*R2) * (R1+R2-+t1*R1-+t2*R2)/(R1+R2-+t1*R1-+t2*R2) =
(R1*R2 +-t1*R1*R2 +-t2*R2*R1 +-t1*t2*R1*R2) /
    (R1+R2+-t1*R1+-t2*R2) * (R1+R2-+t1*R1-+t2*R2)/(R1+R2-+t1*R1-+t2*R2) =


1 / (1/(R1+-t1)+1/(R2+-t2)) =
1 / ((R1-+t1)/(R1^2-t1^2) + (R2-+t2)/(R2^2-t2^2)) =