a) \(x^2+y^2=0\)  ( 1 ) 

Ta có : 

\(x^2\ge0\forall x\)                                                                 

\(y^2\ge0\forall x\)     

Để ( 1 ) = 0 

\(\Rightarrow\hept{\begin{cases}x^2=0\\y^2=0\end{cases}}\)    

\(\hept{\begin{cases}x=0\\y=0\end{cases}}\)    

\(x^2+y^2=0\)   với \(x=y=0\) là mệnh đề đúng 

\(x^2+y^2=0\)  với \(\orbr{\begin{cases}x\ne0\\y\ne0\end{cases}}\)  là mệnh đề sai 

b) \(x^2+y^2\ne0\) ( 2 ) 

Vì \(x^2\ge0\forall x\) 

\(y^2\ge0\forall y\)   

Nên \(x^2+y^2\ne0\Leftrightarrow\orbr{\begin{cases}x^2\ne0\\y^2\ne0\end{cases}}\)    

\(\orbr{\begin{cases}x\ne0\\y\ne0\end{cases}}\) 

\(x^2+y^2\ne0\)    với \(\orbr{\begin{cases}x\ne0\\y\ne0\end{cases}}\) là mệnh đề đúng 

\(x^2+y^2\ne0\)    với \(\hept{\begin{cases}x=0\\y=0\end{cases}}\) là mệnh đề sai 

đéo bít

đếm đi rồi biết

"Giúp tôi giải toán" trên Online Math đã trở thành một diễn đàn hết sức sôi động cho các bạn học sinh, các thầy cô giáo và các bậc phụ huynh từ mọi miền đất nước. Ở đây các bạn có thể chia sẻ các bài toán khó, lời giải hay và giúp nhau cùng tiến bộ. Để diễn đàn này ngày càng hữu ích, các bạn lưu ý các thông tin sau đây:

I. Nội qui tham gia "Giúp tôi giải toán"

1. Không đưa câu hỏi linh tinh lên diễn đàn, chỉ đưa các bài mà mình không giải được hoặc các câu hỏi hay lên diễn đàn;

2. Không trả lời linh tinh, không phù hợp với nội dung câu hỏi trên diễn đàn.

3. Không "Đúng" vào các câu trả lời linh tinh nhằm gian lận điểm hỏi đáp.

Các bạn vi phạm 3 điều trên sẽ bị giáo viên của Online Math trừ hết điểm hỏi đáp, có thể bị khóa tài khoản hoặc bị cấm vĩnh viễn không đăng nhập vào trang web.

II. Cách nhận biết câu trả lời đúng

Trên diễn đàn có thể có rất nhiều bạn tham gia giải toán. Vậy câu trả lời nào là đúng và tin cậy được? Các bạn có thể nhận biết các câu trả lời đúng thông qua 6 cách sau đây:

1. Lời giải rõ ràng, hợp lý (vì nghĩ ra lời giải có thể khó nhưng rất dễ để nhận biết một lời giải có là hợp lý hay không. Chúng ta sẽ học được nhiều bài học từ các lời giải hay và hợp lý, kể cả các lời giải đó không đúng.)

2. Lời giải từ các giáo viên của Online Math có thể tin cậy được (chú ý: dấu hiệu để nhận biết Giáo viên của Online Math là các thành viên có gắn chứ "Quản lý" ở ngay sau tên thành viên.)

3. Lời giải có số bạn chọn "Đúng" càng nhiều thì càng tin cậy.

4. Người trả lời có điểm hỏi đáp càng cao thì độ tin cậy của lời giải sẽ càng cao.

5. Các bài có dòng chữ "Câu trả lời này đã được Online Math chọn" là các lời giải tin cậy được (vì đã được duyệt bởi các giáo viên của Online Math.)

6. Các lời giải do chính người đặt câu hỏi chọn cũng là các câu trả lời có thể tin cậy được.

Chu vi mảnh vườn hình chữ nhật đó là:

30.4=120(m)

Nữa chu vi mảnh vườn hình chữ nhật đó là:

120:2=60(m)

Chiều dài của hình chữ nhật đó là:

30.\(\frac{3}{4}\)=22,5(m)

Chiều rộng hình chữ nhật đó là:

60-22,5=37,5(m)

Diện tích mảnh vườn hình chữ nhật đó là:

22,5.37,5=843,75(m2)

843,75 gấp 100 số lần là:

843,75:100=8,4375( lần)

Trên mảnh vườn đó ,thu hoạch đươc số kg dưa hấu  là:

8,4375.350=2953,125(kg)

                  Đ/s :2953,125 kg dưa hấu

đúng hông zậy bạn ?

2,3 ko chia đc 785,457

12,66 - 2,3 = 10.36

                                         Bài làm :

•  Cách 1:  x²- 6x + 8 

                          = x² - 2x - 4x + 8

                          = x (x - 2) - 4(x -2)

                          = (x - 4)(x -2)

• Cách 2: x² - 6x + 8  

                     = x² - 6x + 9 - 1

                     = ( x - 3)² - 1

                     =( x -3 - 1)( x- 3 + 1)

                     = (x - 4)(x -2)

•  Cách 3: x² - 6x + 8  

                       = x² - 16 - 6x + 24

                       =( x - 4)(x + 4 ) - 6 (x - 4)

                       =(x - 4)(x + 4 - 6)

                       = (x - 4)(x -2) 

Chúc bạn học tốt !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

mình cũng được tròn 3 cách 

c1 \(x^2-6x+8=x^2-2x-4x+8=x\left(x-2\right)-4\left(x-2\right)=\left(x-4\right)\left(x-2\right)\)

c2 \(x^2-6x+8=\left(x^2-6x+9\right)-1=\left(x-3\right)^2-1=\left(x-4\right)\left(x-2\right)\)

c3 Gỉa sử \(x^2-6x+8=\left(x+a\right)\left(x+b\right)=x^2+\left(a+b\right)x+ab\)

Cân bằng hệ số ta được \(\hept{\begin{cases}a+b=-6\\ab=8\end{cases}< =>\orbr{\begin{cases}a=-4\\b=-2\end{cases}or\orbr{\begin{cases}a=-2\\b=-4\end{cases}}}}\)

Vậy ta có : \(\left(x+a\right)\left(x+b\right)=\left(x-2\right)\left(x-4\right)\)

Đề bài là gì thế bạn nhờ?

lm trên symbolab.com

\(\left(2\sin x-1\right)\left(2\sin2x+1\right)=3-4\cos^2x\)

\(\Leftrightarrow\left(2\sin x-1\right)\left(2\sin2x+1\right)=3-4\left(2-\sin^2x\right)\)

\(\Leftrightarrow\left(2\sin x-1\right)\left(2\sin2x+1\right)=4\sin^2x-1\)

\(\Leftrightarrow\left(2\sin x-1\right)\left(2\sin2x+1\right)=\left(2\sin x-1\right)\left(2\sin x+1\right)\)

\(\Leftrightarrow2\sin2x+1=2\sin x+1\)

\(\Leftrightarrow\sin2x=\sin x\)

\(\Leftrightarrow\sin2x-\sin x=0\)

\(\Leftrightarrow2\cos\frac{3}{2}-\cos\frac{x}{2}=0\)

\(\Leftrightarrow\orbr{\begin{cases}\cos\frac{3}{2}=0\\\cos\frac{x}{2}=0\end{cases}}\Leftrightarrow\orbr{\begin{cases}\frac{3x}{2}=\frac{\pi}{2}+k2\pi\\\frac{x}{2}=\frac{\pi}{2}+k2\pi\end{cases}\left(k\inℤ\right)}\)

\(\Leftrightarrow\orbr{\begin{cases}x=\frac{\pi}{3}+\frac{2\pi}{3}k\\x=\pi+4k\pi\end{cases}\left(k\inℤ\right)}\)

a)

\(A=\left(100-99\right)\left(100+99\right)+\left(98-97\right)\left(98+97\right)+...+\left(2-1\right)\left(2+1\right)\)

\(A=100+99+98+97+...+2+1\)

\(A=\frac{100.101}{2}=5050\)

b)

\(B=\left(2^2-1\right)\left(2^2+1\right)\left(2^4+1\right)...\left(2^{64}+1\right)+1\)

\(B=\left(2^4-1\right)\left(2^4+1\right)...\left(2^{64}+1\right)+1\)

\(B=\left(2^8-1\right)...\left(2^{64}+1\right)+1\)

\(B=\left(2^{64}-1\right)\left(2^{64}+1\right)+1\)

\(B=2^{128}-1+1=2^{128}\)

c)

\(C=a^2+b^2+c^2+2\left(ab+bc+ca\right)+a^2+b^2+c^2+2ab-2ac-2bc-2a^2-4ab-2b^2\)

\(C=2c^2\)

thanks bạn nhaaa :3

chú ý phải làm dài

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2 / 3 x 3 / 4 = 2x3 / 3x4 =  6/12 = 1/2 

Học tốt ^_^

ah nha

\(\frac{2}{3}\).\(\frac{3}{4}\)=\(\frac{6}{12}\)=\(\frac{1}{2}\)