a.
\(\begin{array}{l}y' = \cos x\\y" = - \sin x\\y''' = - \cos x\end{array}\)
b.
\(\begin{array}{l}y = \frac{1}{2}\left( {\cos 4x - \cos 6x} \right)\\y' = - 2\sin 4x + 3\sin 6x\\y" = - 8\cos 4x + 18\cos 6x\\y'" = 32\sin 4x - 108\sin 6x\\{y^{\left( 4 \right)}} = 128\cos 4x - 648\cos 6x\end{array}\)
c.
\(\begin{array}{l}y' = - 5{\left( {4 - x} \right)^4}\\y" = 20{\left( {4 - x} \right)^3}\\y"' = - 60{\left( {4 - x} \right)^2}\\{y^{\left( 4 \right)}} = 120\left( {4 - x} \right)\\{y^{\left( 5 \right)}} = - 120\\{y^{\left( n \right)}} = 0\,\left( {\forall n \ge 6} \right)\end{array}\)
d.
\(\begin{array}{l}y = \frac{1}{{x + 2}} = {\left( {x + 2} \right)^{ - 1}}\\y' = - 1{\left( {x + 2} \right)^{ - 2}}\\y" = \left( { - 1} \right)\left( { - 2} \right){\left( {x + 2} \right)^{ - 3}},...\end{array}\)
Bằng qui nạp ta chứng minh được :
\({y^{\left( n \right)}} = \left( { - 1} \right)\left( { - 2} \right)...\left( { - n} \right).{\left( {x + 2} \right)^{ - n - 1}}\)
\(= {\left( { - 1} \right)^n}.\frac{{n!}}{{{{\left( {x + 2} \right)}^{n + 1}}}}\)
e.
\(\begin{array}{l}y = {\left( {2x + 1} \right)^{ - 1}}\\y' = \left( { - 1} \right)\left( {2{{\left( {2x + 1} \right)}^{ - 2}}} \right)\\y" = \left( { - 1} \right)\left( { - 2} \right){.2^2}{\left( {2x + 1} \right)^{ - 3}},...\end{array}\)
Bằng qui nạp ta chứng minh được :
\({y^{\left( n \right)}} = {\left( { - 1} \right)^n}.\frac{{{2^n}.n!}}{{{{\left( {2x + 1} \right)}^{n + 1}}}}\)
f. Ta có:
\(\begin{array}{l}y' = - \sin 2x\\y" = - 2\cos 2x\\y"' = {2^2}\sin 2x\\{y^{\left( 4 \right)}} = {2^3}\cos 2x\\{y^{\left( 5 \right)}} = - {2^4}\sin 2x\\{y^{\left( 6 \right)}} = - {2^5}\cos 2x,...\end{array}\)
Bằng qui nạp ta chứng minh được :
\({y^{\left( {2n} \right)}} = {\left( { - 1} \right)^n}{.2^{2n - 1}}\cos 2x\)
