Câu hỏi 16

\(\begin{align}   y'\left( x \right)=-\sin x \\   y''\left( x \right)=-\cos x \\   y'''\left( x \right)=\sin x \\   {{y}^{\left( 4 \right)}}\left( x \right)=\cos x=y \\   {{y}^{\left( 5 \right)}}\left( x \right)=-\sin x=y' \\   {{y}^{\left( 6 \right)}}\left( x \right)=-\cos x=y'' \\   {{y}^{\left( 7 \right)}}\left( x \right)=\sin x=y''' \\   .... \\ \end{align}\)

Ta có: \(2018=504.4+2\Rightarrow {{y}^{\left( 2018 \right)}}\left( x \right)=y''\left( x \right)=-\cos x\)

Chọn A.