trigonometric

the above graph shows the relationship between love and time. it has a minimum gradient at t=y.

for t > 0,
f(t) > 0

for x > t > y,
f(t) > 0
f'(t) < 0
f"(t) < 0

for t > y,
f(t) > 0
f'(t) < 0
f"(t) > 0