Yes. Let
f(t,y)=cos(ty). Note that
dydcos(ty)=−tsin(ty)
for any
t. The
maximum magnitude of this is
t, meaning that for any change
y2=y1+Δy
the change-of-image
f(y2)−f(y1) will have magnitude no greather than
t⋅Δy; ie,
∣f(y2)−f(y1)∣≤∣t(y2−y1)∣
since we know
t∈[0,1], then this entails
∣f(y2)−f(y1)∣≤1⋅∣y2−y1∣
meaning that
f satisfies a Lipschitz condition in
y with constant
L=1.