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1.
\(y'=m-3cos3x\)
Hàm đồng biến trên R khi và chỉ khi \(m-3cos3x\ge0\) ; \(\forall x\)
\(\Leftrightarrow m\ge3cos3x\) ; \(\forall x\)
\(\Leftrightarrow m\ge\max\limits_{x\in R}\left(3cos3x\right)\)
\(\Leftrightarrow m\ge3\)
2.
\(y'=1-m.sinx\)
Hàm đồng biến trên R khi và chỉ khi:
\(1-m.sinx\ge0\) ; \(\forall x\)
\(\Leftrightarrow1\ge m.sinx\) ; \(\forall x\)
- Với \(m=0\) thỏa mãn
- Với \(m< 0\Rightarrow\dfrac{1}{m}\le sinx\Leftrightarrow\dfrac{1}{m}\le\min\limits_R\left(sinx\right)=-1\)
\(\Rightarrow m\ge-1\)
- Với \(m>0\Rightarrow\dfrac{1}{m}\ge sinx\Leftrightarrow\dfrac{1}{m}\ge\max\limits_R\left(sinx\right)=1\)
\(\Rightarrow m\le1\)
Kết hợp lại ta được: \(-1\le m\le1\)
Đáp án A
Áp dụng công thức trung tuyến ta có:
Theo Định lý PiTaGo ta có:
⇒ E F = 2 b 2
Nhận xét: Nếu x = 1 không là nghiệm của phương trình (1) thì x = 1 là nghiệm đơn của phương trình f(x) = 0 nên f(x) đổi dấu khi qua nghiệm x = 1.
Chọn C
=7986575756465353578708806867463524243445568679988087685746353524132435645675 OK
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