\(2sin3x+1=0\Leftrightarrow sin3x=-\dfrac{1}{2}\)

\(\Leftrightarrow\left[{}\begin{matrix}3x=-\dfrac{\pi}{6}+k2\pi\\3x=\dfrac{7\pi}{6}+n2\pi\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=-\dfrac{\pi}{18}+\dfrac{k2\pi}{3}\\x=\dfrac{7\pi}{18}+\dfrac{n2\pi}{3}\end{matrix}\right.\)

\(x\in\left(-\dfrac{25\pi}{18};\dfrac{31\pi}{18}\right)\Rightarrow\left\{{}\begin{matrix}-\dfrac{25\pi}{18}< -\dfrac{\pi}{18}+\dfrac{k2\pi}{3}\le< \dfrac{31\pi}{18}\\-\dfrac{25\pi}{18}< \dfrac{7\pi}{18}+\dfrac{n2\pi}{3}< \dfrac{31\pi}{18}\end{matrix}\right.\)

\(\Rightarrow\left\{{}\begin{matrix}-2< k< \dfrac{8}{3}\\-\dfrac{8}{3}< n< 2\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}k=\left\{-1;0;1;2\right\}\\n=\left\{-2;-1;0;1\right\}\end{matrix}\right.\)

\(\Rightarrow\) Pt có 8 nghiệm trên khoảng đã cho

ko hiểu

16, a

17, a

18, d

19, a

20, b

21, c

17 B: (động vật hoang dã)

19 A:(khí hậu mưa)

20 B(bị động và expect+to-v)

21 C(động lực)

1.she spends 20 minutes to draw a picture

2.Lan is the most creative in the group

3.No one is smaller than him

4.My pen isn't the same than mine

5.Lan is jealous than her

6.My brother no longer raise pets

-chúc bạn thật thông minh!!!!

6:

\(2^{225}=\left(2^3\right)^{75}=8^{75}\)

\(3^{150}=\left(3^2\right)^{75}=9^{75}\)

mà 8<9

nên \(2^{225}< 3^{150}\)

4: \(\left|5x+3\right|>=0\forall x\)

=>\(-\left|5x+3\right|< =0\forall x\)

=>\(-\left|5x+3\right|+5< =5\forall x\)

Dấu = xảy ra khi 5x+3=0

=>x=-3/5

1:

\(\left(2x+1\right)^4>=0\)

=>\(\left(2x+1\right)^4+2>=2\)

=>\(M=\dfrac{3}{\left(2x+1\right)^4+2}< =\dfrac{3}{2}\)

Dấu = xảy ra khi 2x+1=0

=>x=-1/2

 \(=>Qthu1=0,2.340000=68000J\)

\(=>Qthu2=2100.0,2.20=8400J\)

\(=>Qtoa=2.4200.25=210000J\)

\(=>Qthu1+Qthu2< Qtoa\)=>đá nóng chảy hoàn toàn

\(=>0,2.2100.20+0,2.340000+0,2.4200.tcb=2.4200\left(25-tcb\right)\)

\(=>tcb=14,5^oC\)

Cho em hỏi ngu tí ạ vậy tcb ở nhưng phép tính trên vứt đi đâu ạ 

mình làm câu dưới r nha