\(tan\alpha=3\)

\(1+tan^2\alpha=\dfrac{1}{cos^2\alpha}\)

\(\Rightarrow cos\alpha=\pm\sqrt{\dfrac{1}{1+tan^2\alpha}}=\pm\sqrt{\dfrac{1}{1+3^2}}=\pm\dfrac{\sqrt{10}}{10}\)

\(\Rightarrow A\)

`tan a =3 <=> (sina)/(cosa) =3 <=> sina=3cosa`

Có: `sin^2a+cos^2a =1`

`<=> (3cosa)^2 + cos^2a =1`

`<=> 10cos^2a =1`

`<=> cosa = \pm \sqrt10/10`

`=>` A.

Bạn tham khảo

3.

\(\overrightarrow{AB}=\left(4;2\right)=2\left(2;1\right)\)

Do đó đường thẳng AB nhận \(\left(-1;2\right)\) là 1 vtpt

4.

\(\overrightarrow{AB}=\left(-a;b\right)\)

\(\Rightarrow\) Đường thẳng AB nhận (b;a) là 1 vtpt

1 Van Mieu-Quoc Tu Giam is a famous historical and cultural ralic in Ha Noi

2 Quoc Tu Giam educated the thousands of talented men for the country

3 Hoi An used to be an important trading center in Southeast Asian.

4 Hoi An was certified as a World Cultural Heritage Site World by UNESCO.

5 Hue ,an ancient capital , is an attractive destination to tourists.

6 Nha Rong Harbour is the place where President Ho Chi Minh decided to go abroad to find out the way of liberation of our country .

7 Ha Noi is the culture and politics center the whole of country.

1 C

2 D

3 B

4 A

5 C

6 was made

7 C

8 A

`(x^2+1)/((3-x)(x+2))>=0(x ne -2,3)`

Vì `x^2+1>0`

`=>(3-x)(x+2)>0`

`=>(x-3)(x+2)<0`

`=>-2<x<3`

Ủa thì chọn gì?

\(sin\left(\dfrac{3\pi}{2}-x\right)+tan^2x=sin\left(\pi+\dfrac{\pi}{2}-x\right)+\dfrac{sin^2x}{cos^2x}=-sin\left(\dfrac{\pi}{2}-x\right)+\dfrac{1-cos^2x}{cos^2x}\)

\(=-cosx+\dfrac{1-cos^2x}{cos^2x}=-a+\dfrac{1-a^2}{a^2}=\dfrac{-a^3-a^2+1}{a^2}\)

\(\Rightarrow\left\{{}\begin{matrix}m=-1\\n=-1\end{matrix}\right.\)

74.

\(cos\left(\dfrac{3\pi}{2}-x\right)+cot^2x=cos\left(\pi+\dfrac{\pi}{2}-x\right)+\dfrac{cos^2x}{sin^2x}=-cos\left(\dfrac{\pi}{2}-x\right)+\dfrac{1-sin^2x}{sin^2x}\)

\(=-sinx+\dfrac{1-sin^2x}{sin^2x}=-a+\dfrac{1-a^2}{a^2}=\dfrac{-a^3-a^2+1}{a^2}\)

\(\Rightarrow m=n=-1\)