That's indeed the solution I was suggesting. One fun thing about doing
that way is that it provides a visual proof of the fact that the sum of
the first N odd numbers is N-squared. The number of small triangles in
each strip is 1, 3, 5, etc., while the total number of congruent small
triangles must be N-squared because of the scaling law of area (imagine
horizontal lines between the rows here):
/\ 1
/\/\ 3
/\/\/\ 5
/\/\/\/\ 7
/\/\/\/\/\ + 9 = 25 = 5*5
The league solution appears to be based on the idea that inside the big
triangle
/\ /\ /\
/ \ / \ / \
/ \ , the strip ______ = / \ - /____\ .
/ \ /______\ /______\
/________\
By the scaling law of area, the shaded strip therefore has
(4/5)^2 - (3/5)^2 = 7/25 of the total area, leaving 18/25 for the
unshaded part [where ^2 means "squared"].
PB