Question

lim(cos(4x)-cos^3(4x))/(3x^2)
х стремится к 0

Answer 1

$\lim\limits _{x \to 0}\frac{cos4x-cos^34x}{3x^2}=\lim\limits _{x \to 0}\frac{cos4x\cdot (1-cos^24x)}{3x^2}=\lim\limits _{x \to 0}\frac{cos4x\cdot sin^24x}{3x^2}=\\\\=\Big [\; sina\sim a\; ,\; a\to 0\; \; \Rightarrow \; \; 4x\to 0\; ,\; sin4x\sim 4x\; \Big ]=\lim\limits _{x \to 0}\frac{cos4x\cdot (4x)^2}{3x^2}=\\\\=\lim\limits _{x \to 0}\frac{cos4x\cdot 16x^2}{3x^2}=\lim\limits _{x \to 0}\frac{cos4x\cdot 16}{3}=\Big [\; cos0=1\; \Big ]=\frac{1\cdot 16}{3}=\frac{16}{3}$