Question

Решите довольно лёгкий предел )​

Answer 1

$\lim\limits _{x \to 0}\dfrac{sin2x-2\, sinx}{x\cdot ln(cos5x)}=\lim\limits _{x \to 0}\dfrac{2\cdot sinx\cdot cosx-2\, sinx}{x\cdot ln(cos5x)}=\lim\limits _{x \to 0}\dfrac{-2\cdot sinx\cdot (1-cosx)}{x\cdot ln(cos5x+1-1)}=\\\\\\=\lim\limits _{x \to 0}\ \dfrac{-2\cdot x\cdot \dfrac{x^2}{2} }{x\cdot ln(1+(cos5x-1))}=\lim\limits _{x \to 0}\ \dfrac{-x^2}{ln(1+(cos5x-1))}=\lim\limits _{x \to 0}\dfrac{-x^2}{cos5x-1}=$

$=\lim\limits _{x \to 0}\ \dfrac{-x^2}{-(1-cos5x)}=\lim\limits _{x \to 0}\ \dfrac{x^2}{\dfrac{(5x)^2}{2}}=\lim\limits _{x \to 0}\ \dfrac{2x^2}{25x^2}=\lim\limits _{x \to 0}\ \dfrac{2}{25}=\dfrac{2}{25}\\\\\\\\\star \ \ sin\alpha (x)\sim \alpha(x)\ ,\ \ \alpha (x)\to 0\ \ \star \\\\\star \ \ 1-cos\alpha (x)\sim \dfrac{\alpha^2(x)}{2}\ ,\ \alpha (x)\to 0\ \ \star \\\\\star \ \ ln(1+\alpha (x))\sim \alpha (x)\ ,\ \alpha (x)\to 0\ \ \star$