giữ lời ko làm chó
gọi A = 6 / ( 5*8 ) + ... + 198 / ( 101 * 200 )
=> A / 2 = 3 / ( 5*8 ) + 11 / ( 8 * 19 ) + ... + 99 / ( 101*200 )
A / 2 = 1/5 - 1/8 + 1/8 - 1/11 + ... + 1 / 101 - 1 / 200
A / 2 = 1/ 5 -1 / 200
A / 2 = 39 /200
A = 39 / 100
đã làm bài này rồi , đúng, giờ thì k hộ cái , ko giết đấy
Ta co:
1/2A= 3/5×8 + 11/8×19 + 12/19×31 +70 /31×101+ 99/101×200
1/2A= 1/5 - 1/8 + 1/8 - 1/19 + 1/19 - 1/31 + 1/31 -1/101 + 1/101 - 1/200
1/2A= 1/5 - 1/200
1/2A= 39/200
A= 39/200 ÷ 1/2
A= 39/100
\(A=\frac{6}{5.8}+\frac{22}{8.19}+\frac{24}{19.31}+\frac{140}{31.101}+\frac{198}{101.200}\)
\(A=2\left(\frac{3}{5.8}+\frac{11}{8.19}+\frac{12}{19.31}+\frac{70}{31.101}+\frac{99}{101.200}\right)\)
\(A=2.\left(\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{19}+\frac{1}{19}-\frac{1}{31}+\frac{1}{31}-\frac{1}{101}+\frac{1}{101}-\frac{1}{200}\right)\)
\(A=2\left(\frac{1}{5}-\frac{1}{200}\right)\)
\(A=2.\frac{39}{200}\)
\(A=\frac{39}{100}\)
