Dùng phương pháp tích phân từng phần để tính các tích phân sau:
\(\begin{array}{l}
a)\int \limits_1^2 {x^5}\ln xdx\\
b)\int \limits_0^1 \left( {x + 1} \right){e^x}dx\\
c)\int \limits_0^\pi {e^x}\cos xdx\\
d)\int \limits_0^{\frac{\pi }{2}} x\cos xdx
\end{array}\)
a) Đặt \(\left\{ \begin{array}{l}
u = lnx\\
dv = {x^5}dx
\end{array} \right. \)
\(\Rightarrow \left\{ \begin{array}{l}
du = \frac{{dx}}{x}\\
v = \frac{{{x^6}}}{6}
\end{array} \right.\)
\(\begin{array}{l}
\int\limits_1^2 {{x^5}} \ln xdx = \left. {\frac{{{x^6}}}{6}\ln x} \right|_1^2 - \frac{1}{6}\int\limits_1^2 {{x^5}} dx\\
= \left. {\left( {\frac{{{x^6}}}{6}\ln x - \frac{{{x^6}}}{{36}}} \right)} \right|_1^2 = \frac{{32}}{3}\ln 2 - \frac{7}{4}
\end{array}\)
b) Đặt
\(\begin{array}{*{20}{l}}
{\left\{ {\begin{array}{*{20}{l}}
{u = x + 1}\\
{dv = {e^x}dx}
\end{array}} \right. \Rightarrow \left\{ {\begin{array}{*{20}{l}}
{du = dx}\\
{v = {e^x}}
\end{array}} \right.}\\
\begin{array}{l}
\int\limits_0^1 {\left( {x + 1} \right)} {e^x}dx\\
= \left. {\left( {x + 1} \right){e^x}} \right|_0^1 - \int\limits_0^1 {{e^x}} dx = e
\end{array}
\end{array}\)
c) Đặt \(I = \int \limits_0^\pi {e^x}\cos xdx\)
Đặt
\(\begin{array}{l}
\left\{ {\begin{array}{*{20}{l}}
{u = {e^x}}\\
{dv = \cos xdx}
\end{array}} \right. \Rightarrow \left\{ {\begin{array}{*{20}{l}}
{du = {e^x}dx}\\
{v = \sin x}
\end{array}} \right.\\
\Rightarrow I = \left. {{e^x}\sin x} \right|_0^\pi - \int\limits_0^\pi {{e^x}} {\rm{sinx}}dx\\
= - \int\limits_0^\pi {{e^x}} {\rm{sinx}}dx
\end{array}\)
Đặt \(\left\{ \begin{array}{l}
u = {e^x}\\
dv = sinxdx
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
du = {e^x}dx\\
v = - \cos x
\end{array} \right.\)
Do đó:
\(\begin{array}{*{20}{l}}
\begin{array}{l}
I = - \left[ {( - {e^x}cosx)|_0^\pi + \int\limits_0^\pi {{e^x}} \cos xdx} \right]\\
= {e^\pi }\cos \pi - {e^0}.\cos 0 - I
\end{array}\\
{ \Rightarrow 2I = - {e^\pi } - 1 \Rightarrow I = - \frac{1}{2}\left( {{e^\pi } + 1} \right)}
\end{array}\)
d) Đặt \(\left\{ \begin{array}{l}
u = x\\
dv = \cos xdx
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
du = dx\\
v = \sin x
\end{array} \right.\)
Do đó:
\(\begin{array}{l}
\int\limits_0^{\frac{\pi }{2}} x \cos xdx = x\sin x|_0^{\frac{\pi }{2}} - \int\limits_0^{\frac{\pi }{2}} {\sin x} dx\\
= (x\sin x + \cos x)|_0^{\frac{\pi }{2}} = \frac{\pi }{2} - 1
\end{array}\)
-- Mod Toán 12
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