a) √2 cos(x - π/4)

= √2.(cosx.cos π/4 + sinx.sin π/4)

= √2.(√2/2.cosx + √2/2.sinx)

= √2.√2/2.cosx + √2.√2/2.sinx

= cosx + sinx (đpcm)

b) √2.sin(x - π/4)

= √2.(sinx.cos π/4 - sin π/4.cosx )

= √2.(√2/2.sinx - √2/2.cosx )

= √2.√2/2.sinx - √2.√2/2.cosx

= sinx – cosx (đpcm).

Đặt \(sinx=t\Rightarrow cosx.dx=dt\) ; \(\left\{{}\begin{matrix}x=\dfrac{\pi}{6}\Rightarrow t=\dfrac{1}{2}\\x=\dfrac{\pi}{2}\Rightarrow t=1\end{matrix}\right.\)

\(I=\int\limits^1_{\dfrac{1}{2}}\dfrac{dt}{1+t}=ln\left|1+t\right||^1_{\dfrac{1}{2}}=ln2-ln\left(\dfrac{3}{2}\right)=-ln3+2ln2\)

\(\Rightarrow ab=-2\)

1.A

2.C

3.B

4.C

a

c

b

c

(x-1)y^2-4(x-1)y

Chọn D.

x 4  + 8x = x( x 3 +8)= x( x 3 + 2 3 ) = x(x + 2)(  x 2 − 2x + 4)

\(\Leftrightarrow1-2sin^2x+\left(2m-3\right)sinx+m-2=0\)

\(\Leftrightarrow2sin^2x-\left(2m-3\right)sinx-m+1=0\)

\(\Leftrightarrow2sin^2x+sinx-2\left(m-1\right)sinx-\left(m-1\right)=0\)

\(\Leftrightarrow sinx\left(2sinx+1\right)-\left(m-1\right)\left(2sinx+1\right)=0\)

\(\Leftrightarrow\left(2sinx+1\right)\left(sinx-m+1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}sinx=-\dfrac{1}{2}\\sinx=m-1\end{matrix}\right.\)

Pt có đúng 2 nghiệm thuộc khoảng đã cho khi và chỉ khi:

\(\left\{{}\begin{matrix}m-1\ne-\dfrac{1}{2}\\-1\le m-1\le1\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}m\ne\dfrac{1}{2}\\0\le m\le2\end{matrix}\right.\)

Để \(cos\left(x-\dfrac{\Omega}{4}\right);sinx;cos\left(x+\dfrac{\Omega}{4}\right)\) là ba số hạng liên tiếp của cấp số nhân thì \(sin^2x=cos\left(x-\dfrac{\Omega}{4}\right)\cdot cos\left(x+\dfrac{\Omega}{4}\right)\)

=>\(sin^2x=\sqrt{2}\left(cosx-sinx\right)\cdot\sqrt{2}\left(cosx+sinx\right)\)

=>\(sin^2x=2cos^2x-2sin^2x\)

=>\(3\cdot sin^2x=2\cdot cos^2x\)

=>\(\dfrac{sin^2x}{cos^2x}=\dfrac{2}{3}\)

=>\(tan^2x=\dfrac{2}{3}\)

=>\(\left[{}\begin{matrix}tanx=\dfrac{\sqrt{6}}{3}\\tanx=-\dfrac{\sqrt{6}}{3}\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=arctan\left(\dfrac{\sqrt{6}}{3}\right)+k\Omega\\x=arctan\left(-\dfrac{\sqrt{6}}{3}\right)+k\Omega\end{matrix}\right.\)