Sliding rheostat into the circuit terminals are connected to the left, how to change the resistance

Sliding rheostat into the circuit terminals are connected to the left, how to change the resistance

When the slide moves to the left and close to the lower terminal, the length of the resistance wire connected to the circuit becomes shorter and the resistance becomes smaller;
conversely:
Slide to the right, away from the lower terminal, the length of resistance wire into the circuit becomes longer, the resistance becomes larger

Resistor R1, R2 and R3 are connected into the circuit as shown in the figure and placed in a box. There are three terminals a, B and C on the surface of the box. Please design an experiment with multi-purpose meter and wire, and determine the resistance value of each resistor through the measurement in a, B and C. It is required to write down the experimental steps and use the measured value to represent the resistance R1, R2 and R3

The experimental steps are as follows: (1) connect BC with wire to measure the resistance X between a and B; (2) connect AB with wire to measure the resistance y between B and C; (3) connect AC with wire to measure the resistance Z between B and C; then there is a solution of 1X = 1r1 + 1r2, 1y = 1r3 + 1r2, 1z = 1r1 + 1r3 to get R1 = 2xyzxy + YZ − XZ, R2 = 2xyzxz + YZ − XY, R3 = 2xyzxy + XZ − YZ ：R1=2xyzxy+yz−xz，R2=2xyzxz+yz−xy，R3=2xyzxy+xz−yz．