a
ab
b
ac
c
↑ one rectangle, height a, width (b + c)
Spread out (expanded)
ab + ac
Rebuilt (factorised)
— press the button —
Area of both pieces =
ab
+
ac
⟵ Pull them together
↻ New pair
↺ Reset
Both pieces share the same height
a
. Pull them together and the shared
a
comes
out the front
of one bracket — that is factorising: the reverse of expanding.