from the question
or sinx.cosx≤c.(sin2x+cos2x)3-3.c.sinxcosx
or (1+3.c)2.sinx.cosx≤2.c
or sin2x≤2.c1+3.c
since -1≤sin2x≤1
or 2.c1+3.c=1
or c=-1
\hspace{-16}$If $\bf{\sin x.\cos x\leq \mathbb{C}\big(\sin^6x+\cos ^6x\big)}$. Then $\bf{\mathbb{C}}$ is
from the question
or sinx.cosx≤c.(sin2x+cos2x)3-3.c.sinxcosx
or (1+3.c)2.sinx.cosx≤2.c
or sin2x≤2.c1+3.c
since -1≤sin2x≤1
or 2.c1+3.c=1
or c=-1