A force-extension calibration line is stronger when every added point shares the same checked slope. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The first calibration point is two metres
Slope 2 newtons per metre maps extension 2 metres to force 4 newtons. A second witness point keeps the line from being a one-point claim.
2⋅2=4
Adding a third point keeps the same line
The same slope maps the added extension 6 metres to force 12 newtons. The diagram now has three checked calibration points.
2⋅6=12
Four calibration points make the slope auditable
Extension 8 metres lands at force 16 newtons. Four displayed points share the same checked slope.