To generalise means to make a statement which holds in all
cases, not just in some particular cases. A generalization
is often expressed algebraically.
The diagram shows that the 4th triangle number
is (4 x 5) / 2 (because two lots of the 4th triangle number
make a 4 by 5 rectangle). The method can be generalized as
: the n th triangle n(n + 1) / 2.