Question

tinh nhanh a, 2/5x8+2/ 8x11 +2/ 11x14 + ... +2/ 95x98

Answer 1

\(A=\dfrac{2}{5\times8}+\dfrac{2}{8\times11}+\dfrac{2}{11\times14}+...+\dfrac{2}{95\times98}\)

\(=2\times\dfrac{1}{3}\times\left(\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{14}+...+\dfrac{1}{95}-\dfrac{1}{98}\right)\)

\(=\dfrac{2}{3}\times\left(\dfrac{1}{5}-\dfrac{1}{98}\right)\)

\(=\dfrac{2}{3}\times\dfrac{93}{490}\)

\(=\dfrac{31}{245}\)

Answer 2

\(\dfrac{2}{5x8}\) + \(\dfrac{2}{8x11}\) + \(\dfrac{2}{11x14}\)+........+\(\dfrac{2}{95x98}\)

Answer 3

Đề sai

Answer 4

2/5x8+2/8x11+2/11x14+...+2/95x98

 

=2(1/5x8+1/8x11+1/11x14+...+1/95x98) (khoảng cách từ 5-8;8-11;11-14;...;95-98 la 3) suy ra =2/3(1/5-1/8+1/8-1/11+1/11-1/14+...+1/95-1/98)

 

=2/3(1/5-1/98)=2/3x93/5x98=31/245

Answer 5

Sai chỗ nào vậy bạn? _{* bài này tính nhanh nhưng hình như là số hơi lố *}

Answer 6

kết quả là \(\dfrac{31}{245}\) cô nhé!

Answer 7

A = \(\dfrac{2}{5\times8}\) + \(\dfrac{2}{8\times11}\)+ \(\dfrac{2}{11\times14}\)+...+ \(\dfrac{2}{95\times98}\)

A = \(\dfrac{2}{3}\)\(\times\)( \(\dfrac{3}{5\times8}\) + \(\dfrac{3}{8\times11}\)+ \(\dfrac{3}{11\times4}\)+...+ \(\dfrac{3}{95\times98}\))

A = \(\dfrac{2}{3}\) \(\times\)( \(\dfrac{1}{5}\) - \(\dfrac{1}{8}\) + \(\dfrac{1}{8}\) - \(\dfrac{1}{11}\)+...+ \(\dfrac{1}{95}\)-\(\dfrac{1}{98}\))

A = \(\dfrac{2}{3}\) \(\times\)( \(\dfrac{1}{5}\) - \(\dfrac{1}{98}\))

A = \(\dfrac{31}{245}\)