Question

Tính các tích phân sau :

a) \(\int\limits^1_0\left(y-1\right)^2\sqrt{y}dy\), đặt \(t=\sqrt{y}\)

b) \(\int\limits^2_1\left(x^2+1\right)\sqrt[3]{\left(z-1\right)^2}dz\), đặt \(u=\sqrt[3]{z-1}\)

c) \(\int\limits^e_1\dfrac{\sqrt{4+5\ln x}}{x}dx\)

d) \(\int\limits^{\dfrac{\pi}{2}}_0\left(\cos^5\varphi-\sin^5\varphi\right)d\varphi\)

e) \(\int\limits^{\pi}_0\cos^3\alpha\cos3\alpha d\alpha\)

Answer 1

Câu a)

Đặt \(y=\sqrt{t}\Rightarrow I_1=\int ^{1}_{0}(y-1)^2\sqrt{y}dy=\int ^{1}_{0}(t^2-1)^2td(t^2)\)

\(\Leftrightarrow I_1=2\int^{1}_{0}(t^2-1)^2t^2dt=2\int ^{1}_{0}(t^6-2t^4+t^2)dt\)

\(=2\left.\begin{matrix} 1\\ 0\end{matrix}\right|\left ( \frac{t^7}{7}-\frac{2t^5}{5}+\frac{t^3}{3} \right )=\frac{16}{105}\)

b) Đặt \(u=\sqrt[3]{z-1}\Rightarrow z=u^3+1\Rightarrow I_2=\int ^{1}_{0}[(u^3+1)^2+1]u^2d(u^3+1)\)

\(\Leftrightarrow I_2=3\int ^{1}_{0}[(u^3+1)^2+1]u^4du=3\int ^{1}_{0}(u^{10}+2u^7+2u^4)du\)

\(=3\left.\begin{matrix} 1\\ 0\end{matrix}\right|\left ( \frac{x^{11}}{11}+\frac{x^8}{4}+\frac{2x^5}{5} \right )=\frac{489}{220}\)

Answer 2

c) Ta có:

\(I_3=\int ^{e}_{1}\frac{\sqrt{4+5\ln x}}{x}dx=\int ^{e}_{1}\sqrt{4+5\ln x}d(\ln x)\)

Đặt \(\sqrt{4+5\ln x}=t\Rightarrow I_3=\int ^{3}_{2}td\left (\frac{t^2-4}{5}\right)=\frac{2}{5}\int ^{3}_{2}t^2dt=\frac{38}{15}\)

d)

Xét \(\int ^{\frac{\pi}{2}}_{0}\cos ^5xdx=\int ^{\frac{\pi}{2}}_{0}\cos ^4xd(\sin x)=\int ^{\frac{\pi}{2}}_{0}(1-\sin ^2x)^2d(\sin x)\)

\(=\int ^{1}_{0}(1-t^2)^2dt\)

Xét \(\int ^{\frac{\pi}{2}}_{0}\sin ^5xdx=-\int ^{\frac{\pi}{2}}_{0}\sin ^4xd(\cos x)=-\int ^{\frac{\pi}{2}}_{0}(1-\cos ^2x)^2d(\cos x)=\int ^{1}_{0}(1-t^2)^2dt\)

Do đó \(\int ^{\frac{\pi}{2}}_{0}(\cos ^5x-\sin ^5x)dx=0\)

e)

Có \(\int \cos ^3x\cos 3xdx=\int \cos 3x\left ( \frac{3\cos x+\cos 3x}{4} \right )dx=\frac{1}{4}\int \cos ^23xdx+\frac{3}{4}\int \cos x\cos 3xdx\)

\(=\frac{1}{8}\int (1+\cos 6x)dx+\frac{3}{8}\int (\cos 4x+\cos 2x)dx\)

\(=\frac{1}{8}\int (1+\cos 6x)dx+\frac{3}{8}\int (\cos 4x+\cos 2x)dx=\frac{x}{8}+\frac{\sin 6x}{48}+\frac{3\sin 4x}{32}+\frac{3\sin 2x}{16}\)

Suy ra \(\int ^{\pi}_{0}\cos ^3x\cos 3xdx=\frac{\pi}{8}\)