B3 :
b/ \(x^3-49x=0\)
\(\Leftrightarrow x\left(x^2-49\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x^2-49=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=0\\x^2=49\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=0\\x=\pm7\end{matrix}\right.\)
Vậy ...
c/ \(\Leftrightarrow5x^2-6x+30-2x-5x^2=0\)
\(\Leftrightarrow-8x=-30\)
\(\Leftrightarrow x=\dfrac{30}{8}\)
Vậy ...
d/ \(\Leftrightarrow x^2+3x+7x+21=0\)
\(\Leftrightarrow x\left(x+3\right)+7\left(x+3\right)=0\)
\(\Leftrightarrow\left(x+7\right)\left(x+3\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x+7=0\\x+3=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=-7\\x=-3\end{matrix}\right.\)
Vậy ...
B4
a/ Xét ΔAHB và ΔAHC có :
AB = AC (gt)
BH = HC (gt)
AH : chung
⇒ ΔAHB = ΔAHC (c.c.c)