The trough is the bottom and the crest is the top.
The whole vertical length of the crest, trough, and crest are called a wavelength.
See related link for a diagram.
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Numerator on top; denominator on the bottom.
the top adds what the bottom adds. the top subtracts what the bottom subtracts. the top divides what the bottom divides. the top multiplies what the bottom multiplies.
bottom
Top: numerator. Bottom: denominator.
Aha! A sparkler burns top to bottom only because you light the top. If you lit the bottom, it would burn bottom to top. If you lit the middle, it would burn both directions from the middle.