If you need to estimate the integrand anyway, I would just go ahead and use the Taylor bounds you mentioned. F(x)=\frac{1}{x}\int_x^{4x} \cos(1/u)\, du\Rightarrow \\ \\ 3=\frac{1}{x}\int_x^{4x}\, du\geq F(x)\geq \frac{1}{x}\int_x^{4x}(1-\frac{1}{2u^2})\, du = 3-\frac{3}{8x^2}\\ \\ \Rightarrow \lim_{x\rightarrow\infty} F(x)=3. lol he didn't mention it that was me inserting my commentary into his quote.