\(A=\frac{1}{4\times7}+\frac{1}{7\times10}+\frac{1}{10\times13}+....+\frac{1}{127\times130}.\)

\(3A=\frac{3}{4\times7}+\frac{3}{7\times10}+\frac{3}{10\times13}+....+\frac{3}{127\times130}\)

\(3A=\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+.....+\frac{1}{127}-\frac{1}{130}\)

\(3A=\frac{1}{4}-\frac{1}{130}\)

\(3A=\frac{63}{260}\)

\(A=\frac{21}{260}\)

\({3\over 10 }\) :)))

 tớ không chắc lắm đâu:

ta có :

SABM=3/8 ABC[ VÌ CÓ CHUNG CHIỀU CAO HẠ TỪ B XUỐNG AC]

  vậy SABM là : 48x3/8=18cm

                           đáp số 

CÁM ƠN BẠN 

\(A=\frac{1}{4\times6}+\frac{1}{6\times8}+\frac{1}{8\times10}+....+\frac{1}{92\times94}.\)

\(2A=\frac{2}{4\times6}+\frac{2}{6\times8}+\frac{2}{8\times10}+....+\frac{2}{92\times94}\)

\(2A=\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+.....+\frac{1}{92}-\frac{1}{94}\)

\(2A=\frac{1}{4}-\frac{1}{94}\)

\(2A=\frac{45}{188}\)

\(A=\frac{45}{376}\)

\(A=\frac{1}{2}\times\left(\frac{2}{4\times6}+\frac{2}{6\times8}+\frac{2}{8\times10}+...+\frac{2}{92\times94}\right)\)

\(A=\frac{1}{2}\times\left(\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+...+\frac{1}{92}-\frac{1}{94}\right)\)

\(A=\frac{1}{2}\times\left(\frac{1}{4}-\frac{1}{94}\right)\)

\(A=\frac{1}{2}\times\frac{45}{188}\)

\(A=\frac{45}{376}\)

16,49,100

7² nhá :)))))

cho a,b,c,d,n thuộc N*; biết ab=cd. Chứng minh rằng: a^n + b^n + c^n + d^n là hợp số.

ab=cd`

`⇔a/c=d/b `

Đặt `a/c=d/b=k`

`⇒a=ck;d=bk `

Ta có:

`A=a^n+b^n+c^n+d^n`

`⇔A=(ck)^n+b^n+c^n+(bk)^n`

`⇔A=c^n . k^n+b^n+c^n+b^n . k^n`

`⇔A=c^n(k^n+1)+b^n(k^n+1)`

`⇔A=(c^n+b^n)(k^n+1)`

`⇒A` là hợp số 

bú