\(A.sin\dfrac{\pi}{7}=sin\left(\dfrac{\pi}{7}\right)cos\left(\dfrac{\pi}{7}\right)cos\left(\dfrac{2\pi}{7}\right)cos\left(\dfrac{4\pi}{7}\right)\)

\(=\dfrac{1}{2}sin\left(\dfrac{2\pi}{7}\right)cos\left(\dfrac{2\pi}{7}\right)cos\left(\dfrac{4\pi}{7}\right)\)

\(=\dfrac{1}{4}sin\left(\dfrac{4\pi}{7}\right)cos\left(\dfrac{4\pi}{7}\right)\)

\(=\dfrac{1}{8}sin\left(\dfrac{8\pi}{7}\right)\)

\(=\dfrac{1}{8}sin\left(\pi+\dfrac{\pi}{7}\right)=\dfrac{1}{8}sin\left(-\dfrac{\pi}{7}\right)\)

\(=-\dfrac{1}{8}sin\left(\dfrac{\pi}{7}\right)\)

\(\Rightarrow A=-\dfrac{1}{8}\)

2/7+3/7:6/7

= 2/7+3/7.7/6

= 2/7+ 1/2

=4/14+7/14

= 11/14

tick chi mik nha

4/7.(8/-13)+4/7(-2/13)+1 3/13(-4/7)

= 4/7(-8/13-2/13+1)

=4/7. 3/13 =12/91

Thanks nha

lol

boss đang gấp mọe như b**

\(B=\left(1-\frac{1}{7}\right)\left(1-\frac{2}{7}\right)\left(1-\frac{3}{7}\right)...\left(1-\frac{2011}{7}\right)\)

\(B=\frac{6}{7}.\frac{5}{7}.\frac{4}{7}....\frac{-2004}{7}\)

Còn lại bạn tự tính đi.

\(=78:225\cdot6\cdot7=\dfrac{364}{25}\)

\(A=\dfrac{7}{1.2}+\dfrac{7}{2.3}+\dfrac{7}{3.4}+...+\dfrac{7}{2011.2012}\)

\(A=7\left(\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+...+\dfrac{1}{2011.2012}\right)\)

\(A=7\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{2011}-\dfrac{1}{2012}\right)\)

\(A=7\left(1-\dfrac{1}{2012}\right)=7.\dfrac{2011}{2012}=\dfrac{14077}{2012}\)

Đ=(12/6+7/8)-(-7/8+12/6)

 Bỏ ngoặc 

\(Đ=\frac{12}{6}+\frac{7}{8}+\frac{7}{8}-\frac{12}{6}\)

\(Đ=\left(\frac{12}{6}-\frac{12}{6}\right)+\left(\frac{7}{8}+\frac{7}{8}\right)\)

\(Đ=0+\frac{7}{4}\)

\(Đ=\frac{7}{4}\)

Xin lỗi mình viết nhầm đề bài

Đ=-(12/6+7/8)-(-7/8+12/6)