\(\Delta'=4m^2-2\left(2m^2-1\right)=2>0\) pt luôn có 2 nghiệm

Theo hệ thức Viet: \(\left\{{}\begin{matrix}x_1+x_2=2m\\x_1x_2=\dfrac{2m^2-1}{2}\end{matrix}\right.\)

Do \(x_1\) là nghiệm nên: \(2x_1^2-4mx_1+2m^2-1=0\Leftrightarrow2x_1^2=4mx_1-2m^2+1\)

Thay vào bài toán:

\(4mx_1-2m^2+1+4mx_2+2m^2-9< 0\)

\(\Leftrightarrow4m\left(x_1+x_2\right)-8< 0\)

\(\Leftrightarrow8m^2-8< 0\)

\(\Rightarrow m^2< 1\Rightarrow-1< m< 1\)

Ta có: \(\widehat{BCD}=\dfrac{1}{2}\widehat{BOD}=20^0\) (góc nội tiếp bằng 1 nửa góc ở tâm cùng chắn BD)

\(\widehat{BEC}=180^0-\widehat{BED}=120^0\)

\(\Rightarrow\widehat{CBA}=180^0-\left(\widehat{BEC}+\widehat{BCD}\right)=40^0\) (tổng 3 góc trong tam giác)

\(\Rightarrow sđ\stackrel\frown{AC}=2\widehat{CBA}=80^0\)

Hình vẽ (chỉ mang tính chất minh họa):

Gọi \(E=BH\cap AC\) => \(EB\perp AC\)

Xét \(\Delta AHE\) và \(\Delta BHD\) có:

\(AH=HD\)

\(\widehat{AHE}=\widehat{BHD}\) (hai góc đối đỉnh)

\(\widehat{AEH}=\widehat{BDH}=90^0\)

=> \(\Delta AHE\)=\(\Delta BHD\) (ch-gn) \(\Rightarrow AE=DH\)

Tam giác ABE vuông tại E có: \(cosA=\dfrac{AE}{AB}=\dfrac{DH}{AB}\)

Tam giác BAD vuông tại D có: \(cosB=\dfrac{BD}{AB}\)

\(\Rightarrow cosA=cosB\) 

Ý D

Do M và N lần lượt là trung điểm của cung AB,AC => \(sđ\stackrel\frown{AN}=sđ\stackrel\frown{NC};sđ\stackrel\frown{AM}=sđ\stackrel\frown{MB}\)

Có \(\widehat{AHK}=\dfrac{1}{2}\left(sđ\stackrel\frown{AN}+sđ\stackrel\frown{MB}\right)=\dfrac{1}{2}\left(sđ\stackrel\frown{NC}+sđ\stackrel\frown{AM}\right)\)\(=\widehat{AKH}\)

=> Tam giác AHK cân tại A

Ý A

Ta có \(\widehat{MAC}=\dfrac{1}{2}sđ\stackrel\frown{MB}=\dfrac{1}{2}sđ\stackrel\frown{MA}=\widehat{MDA}\Rightarrow\Delta MAC\sim\Delta MDA\left(g.g\right)\Rightarrow MA^2=MC.MD=\left(12\right)^2\Rightarrow MA=12\).

Chọn C

Kẻ đường cao AH. Khi đó \(BH=CH=a\).

Ta có \(CD.CA=CH.CB\Rightarrow CD=\dfrac{2a^2}{b}\Rightarrow AD=\left|AC-CD\right|=\left|b-\dfrac{2a^2}{b}\right|=\dfrac{\left|b^2-2a^2\right|}{b}\)

\(\left\{{}\begin{matrix}mx-y=3\\2x+my=9\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}y=mx-3\left(1\right)\\2x+my=9\left(2\right)\end{matrix}\right.\)

Thay (1) vào (2)\(\Rightarrow2x+m\left(mx-3\right)=9\)\(\Leftrightarrow x\left(2+m^2\right)=9+3m\) \(\Leftrightarrow x=\dfrac{9+3m}{2+m^2}\)

\(\Rightarrow y=mx-3=\dfrac{m\left(9+3m\right)}{2+m^2}-3=\dfrac{9m-6}{2+m^2}\)

\(P=3x-y=\dfrac{3\left(9+3m\right)}{2+m^2}-\dfrac{9m-6}{2+m^2}\)\(=\dfrac{33}{2+m^2}\)

Để \(P\in Z\Leftrightarrow2+m^2\in Z\)  \(\Rightarrow2+m^2\inƯ\left(33\right)\) mà \(m^2+2\ge2\forall m\) \(\Rightarrow2+m^2\inƯ\left(33\right)=\left\{11;33\right\}\)

TH1: \(2+m^2=11\Leftrightarrow m^2=9\Leftrightarrow\left[{}\begin{matrix}m=3\left(tm\right)\\m=-3\left(L\right)\end{matrix}\right.\)

TH2:\(2+m^2=33\Leftrightarrow m^2=31\Leftrightarrow\left[{}\begin{matrix}m=\sqrt{31}\\m=-\sqrt{31}\end{matrix}\right.\)(ktm)

=> Có 1 giá trị => Ý A

Gọi \(J=CE\cap AB\), \(F=BD\cap AC\) , \(H=CE\cap BD\)

Có \(\widehat{EAB}=\widehat{ECB}=\dfrac{1}{2}sđ\stackrel\frown{EB}\)

\(\widehat{CAD}=\widehat{DBC}=\dfrac{1}{2}sđ\stackrel\frown{DC}\)

\(\Rightarrow\widehat{EAB}+\widehat{CAD}=\widehat{ECB}+\widehat{DBC}=180^0-\widehat{BHC}\)  (*)

Lại có \(\widehat{AJC}+\widehat{AFB}=180^0\) => Tứ giác AJHF nội tiếp đường tròn

\(\Rightarrow180^0=\widehat{BAC}+\widehat{JHF}=\widehat{BAC}+\widehat{BHC}\)

\(\Rightarrow180^0-\widehat{BHC}=\widehat{BAC}\) (2*)

Từ (*); (2*) => \(\widehat{EAB}+\widehat{CAD}=\widehat{BAC}\)

\(\Leftrightarrow\widehat{EAB}+\widehat{BAC}+\widehat{CAD}=2\widehat{BAC}\)

\(\Leftrightarrow\widehat{EAD}=2\alpha\)

Ý C