19: Sine is
2π-periodic,
by the rules the first term is
π/4-periodic.
Cosine is
2π-periodic,
by the rules the first term is
π/6-periodic.
The "least common multiple" (that is, the smallest number T such that
T/(π/4)
and
T/(π/6)
are both integers)
is
T = π/2.
![](gif2/ecb2bg2.gif)
Answer