Bài 2:
Ta có:
$\begin{cases}u_{54}=u_{1}+53d=-61\\u_{4}=u_{1}+3d=64\end{cases}$
$⇔\begin{cases}u_{1}=71,5\\d=-2,5\end{cases}$
$⇒u_{25}=u_{1}+24d=71,5+24.(-2,5)=11,5$
Bài 3:
Ta có: $u_{1}+u_{2}+u_{3}=9$
$⇔u_{1}+u_{1}+d+u_{1}+2d=9$
$⇔3u_{1}+3d=9$
$⇔u_{1}+d=3$
Ta có: $u_{3}^{2}+u_{5}^{2}=4$
$⇔(u_{1}+2d)^{2}+(u_{1}+4d)^{2}=4$
$⇔(3+d)^{2}+(3+3d)^{2}=4$
$⇔9+6d+d^{2}+9+18d+9d^{2}=4$
$⇔10d^{2}+24d+14=0$
$⇔\left[ \begin{array}{l}d=-1\\d=-1,4\end{array} \right.$
Bạn làm tiếp nha
