Question

Интегральные исчесления подстановки
1)xdx/корень a^2-x^2
2)2x+1/x^2+x+1 dx

Answer 1

$1.~~\displaystyle \int {\dfrac{x~dx}{\sqrt{a^2-x^2}}=\int\dfrac{-2x~dx}{-2\sqrt{a^2-x^2}}=-\dfrac12\int\dfrac{d(-x^2)}{\sqrt{a^2-x^2}}=-\dfrac12\int\dfrac{dt}{\sqrt{a^2+t}} ~~((t=-x^2))~=$$=\displaystyle -\dfrac12 \int \dfrac{d(t+a^2)}{\sqrt{t+a^2}}=-\dfrac12\int\dfrac{ds}{\sqrt s}~~ ((s=t+a^2))~=-\dfrac12\cdot2\cdot\sqrt s=-\sqrt{a^2+x^2}+const$

$2. \displaystyle\int\dfrac{2x+1}{x^2+x+1} dx=\int \dfrac{u'(x)dx}{u(x)}~((u(x)=x^2+x+1))~=\int\dfrac{du}u=\ln u+const=\ln(x^2+x+1)+const$