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\(3xC=\frac{3}{1x2x3x4}+\frac{3}{2x3x4x5}+...+\frac{3}{97x98x99x100}\)
\(3xC=\frac{4-1}{1x2x3x4}+\frac{5-2}{2x3x4x5}+...+\frac{100-97}{97x98x99x100}\)
\(3xC=\frac{1}{1x2x3}-\frac{1}{2x3x4}+\frac{1}{2x3x4}+\frac{1}{3x4x5}+...+\frac{1}{97x98x99}-\frac{1}{98x99x100}\)
\(3xC=\frac{1}{1x2x3}-\frac{1}{98x99x100}\Rightarrow C=\frac{\frac{1}{6}-\frac{1}{98x99x100}}{3}\)
a) (-124) + 24 = -100
b) 37. 78 + 37. 22
= 37 . ( 78 + 22 )
= 37 . 100
= 3700
Bài 1:
a, 386-287 -386 -13
= (386-386) -287-13
= 0 -300
=-300
b, =332-681 -232+431
= (332 -232) +(431-681)
= 100 - 250
= -150
Bài 2
a, = 27+65+ 364-27-65
= (27-27) +(65-65) +364
= 0+0+ 364
=364
b, = 42-69+17 -42-17
= (42-42) + (17-17) -69
= -69
chúc bạn học tốt nha!
a: \(=\dfrac{-5}{7}-\dfrac{2}{7}+\dfrac{3}{4}+\dfrac{1}{4}-\dfrac{1}{5}=-\dfrac{1}{5}\)
b: \(=\dfrac{-3}{31}-\dfrac{28}{31}+\dfrac{-6}{17}-\dfrac{11}{17}+\dfrac{1}{29}-\dfrac{1}{5}=\dfrac{-24}{145}\)
\(3B=\dfrac{3}{1\cdot2\cdot3\cdot4}+\dfrac{3}{2\cdot3\cdot4\cdot5}+...+\dfrac{3}{27\cdot28\cdot29\cdot30}\)
\(=\dfrac{1}{1\cdot2\cdot3}-\dfrac{1}{2\cdot3\cdot4}+\dfrac{1}{2\cdot3\cdot4}-\dfrac{1}{3\cdot4\cdot5}+...+\dfrac{1}{27\cdot28\cdot29}-\dfrac{1}{28\cdot29\cdot30}\)
\(=\dfrac{1}{1\cdot2\cdot3}-\dfrac{1}{28\cdot29\cdot30}\)
\(=\dfrac{1353}{8120}\)
=>B=451/8120
thank