\(A=4x^2+4x+1-9x^2+12x-4+x-11=-5x^2+17x-14=-5\left(x^2-\dfrac{17}{5}x+\dfrac{289}{100}\right)+\dfrac{9}{20}=-5\left(x-\dfrac{17}{10}\right)^2+\dfrac{9}{20}\le\dfrac{9}{20}\)

\(maxA=\dfrac{9}{20}\Leftrightarrow x=\dfrac{17}{10}\)

a) \(\Rightarrow x^2\left(x^2-64\right)=0\Rightarrow x^2\left(x-8\right)\left(x+8\right)=0\)

\(\Rightarrow\left[{}\begin{matrix}x=0\\x=8\\x=-8\end{matrix}\right.\)

b) \(\Rightarrow x^2\left(x-6\right)+3x\left(x-6\right)+21\left(x-6\right)=0\Rightarrow\left(x-6\right)\left(x^2+3x+21\right)=0\)

\(\Rightarrow x=6\)

14C  15D  16D

Gọi quãng đường AB là S(km)(S>0)

\(\left\{{}\begin{matrix}t_1=\dfrac{S}{20}\left(h\right)\\t_2=\dfrac{S}{30}\left(h\right)\end{matrix}\right.\)

\(v_{tb}=\dfrac{S+S}{t_1+t_2}=\dfrac{2S}{\dfrac{S}{20}+\dfrac{S}{30}}=\dfrac{2S}{S\left(\dfrac{1}{20}+\dfrac{1}{30}\right)}=\dfrac{2}{\dfrac{1}{12}}=24\left(\dfrac{km}{h}\right)\)

Máy e lag đó trùi =))

\(p=\dfrac{F}{S}=\dfrac{P}{S}=\dfrac{10.m}{S}=\dfrac{10.14,4}{0,2^2}=3600\left(Pa\right)\)

\(=\left(\sqrt{x}+\sqrt{y}\right)\left(\sqrt{x}-\sqrt{y}\right)-\left(\sqrt{x}+\sqrt{y}\right)=\left(\sqrt{x}+\sqrt{y}\right)\left(\sqrt{x}-\sqrt{y}-1\right)\)

\(F=P=10m=10.15=150\left(N\right)\)

\(1h30ph=1,5h\)

\(S=v.t=50.1,5=75\left(km\right)\left(D\right)\)