23:
u4=10 và u7=22
=>\(\left\{{}\begin{matrix}u1+3d=10\\u1+6d=22\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-3d=-12\\u1+3d=10\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}d=4\\u1=10-12=-2\end{matrix}\right.\)
=>Chọn C
Câu 22:
\(\left\{{}\begin{matrix}u1+2u5=0\\S_4=14\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}u1+2\left(u1+4d\right)=0\\4\cdot\dfrac{\left[2u1+3d\right]}{2}=14\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}3u1+8d=0\\2u1+3d=7\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}6u1+16d=0\\6u1+9d=21\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}7d=-21\\2u_1+3d=7\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}d=-3\\2u_1=7-3d=7+9=16\end{matrix}\right.\)
=>\(u_1=8;d=-3\)
=>Chọn A
21A
19B
