cho D= 1/1x4+1/4x7+1/7x10+...+1/91.94
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tính nhanh 1/1x4 + 1/4x7 +1/7x10 +...+ 1/37x40
Các bạn giúp mình với
\(A=\dfrac{1}{1\times4}+\dfrac{1}{4\times7}+...+\dfrac{1}{37\times40}\\ =\dfrac{1}{3}\times\left(\dfrac{3}{1\times4}+\dfrac{3}{4\times7}+...+\dfrac{3}{37\times40}\right)\\ =\dfrac{1}{3}\times\left(1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+...+\dfrac{1}{37}-\dfrac{1}{40}\right)\\ =\dfrac{1}{3}\times\left(1-\dfrac{1}{40}\right)\\ =\dfrac{1}{3}\times\dfrac{39}{40}\\ =\dfrac{13}{40}\)
1/1x4 + 1/4x7 + 1/7x10 +............+ 1/16x19 + 1/19x22
ai đúng mình bấm cho
=1−14 +14 −110 +...+119 −122
=1−122
= \(\frac{21}{22}\)
k cho mình nha chắc chắn đúng 100 %
\(=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{10}+...+\frac{1}{19}-\frac{1}{22}\)
\(=1-\frac{1}{22}\)
\(=\frac{21}{22}\)
\(=\frac{1}{3}\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{16.19}+\frac{3}{19.22}\right)\)
\(=\frac{1}{3}\left(\frac{1}{1}-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{20}\right)\)
\(=\frac{1}{3}\left(1-\frac{1}{20}\right)\)
\(=\frac{1}{3}.\frac{19}{20}\)
\(=\frac{19}{60}\)
\(\frac{1}{1x4}+\frac{1}{4x7}+\frac{1}{7x10}+.......+\frac{1}{77x80}\)
giúp mình nha
Ai nhanh mình sẽ tick
\(=\frac{1}{3}x\left(\frac{3}{1x4}+\frac{3}{4x7}+...+\frac{3}{77x80}\right)\)
\(=\frac{1}{3}x\left(\frac{1}{1}-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+...+\frac{1}{77}-\frac{1}{80}\right)\)
\(=\frac{1}{3}x\left(\frac{1}{1}-\frac{1}{80}\right)\)
\(=\frac{1}{3}\times\frac{79.}{80}\)
\(=\frac{79}{240}\)
Tk giúp mk nha cảm ơn !!
3/1x4+3/4x7+3/7x10+...3/197x200
Ai nhanh tay sẽ được mình tick !!!
\(A=\frac{3}{1\times4}+\frac{3}{4\times7}+\frac{3}{7\times10}+....+\frac{3}{197\times200}\)
\(A=\frac{1}{1\times4}+\frac{1}{4\times7}+\frac{1}{7\times10}+.....+\frac{1}{197\times200}\)
\(A=\frac{1}{1}-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+....\frac{1}{197}-\frac{1}{200}\)
\(A=\frac{1}{1}-\frac{1}{200}\)
\(A=\frac{199}{200}\)
a = 1/1- 1 /197 - 1 /200
a = 1/1 - 1/200
a = 199/200
tinh nhanh 1/1x4 + 1/4x7 +1/7x10 +...+ 1/91x94
Ta có :
1/1.4+1/4.7+...+1/91.94
=1/3.(1/1-1/4+...+1/91-1/94)
=1/3.(1/1-1/94)
=1/3.93/94
=31/94
Ta có :
1/1.4+1/4.7+...+1/91.94
=1/3.(1/1-1/4+...+1/91-1/94)
=1/3.(1/1-1/94)
=1/3.93/94
=31/94
Ta có :
1/1.4+1/4.7+...+1/91.94
=1/3.(1/1-1/4+...+1/91-1/94)
=1/3.(1/1-1/94)
=1/3.93/94
=31/94
Các bạn giúp mình với : 3/1x4 + 3/4x7 + 3/7x10 + 3/10x13
Tính nhanh
A=3²/1x4+3² /4x7+3²/7x10+3²/11x12+3²/13x16+. .... +3²/97x100
Giúp mình với nhé
tinh nhanh 1/1x4 + 1/4x7 +1/7x10 +...+ 1/91x94
tinh nhanh 1/1x4 + 1/4x7 +1/7x10 +...+ 1/91x94
Ta có :
1/1.4+1/4.7+...+1/91.94
=1/3.(1/1-1/4+...+1/91-1/94)
=1/3.(1/1-1/94)
=1/3.93/94
=31/94
tinh nhanh 1/1x4 + 1/4x7 +1/7x10 +...+ 1/91x94
1/1.4+1/4.7+1/7.10+...+1/91.94
=1/3.(3/1.4+3/4.7+3/7.10+...+3/91.94)
=1/3.(1-1/4+1/4-1/7+1/7-1/10+...+1/91-1/94)
=1/3.(1-1-94)
=1/3.(93/94)
=31/94
Đặt A= 1/1*4+1/4*7+1/7*10+....+1/91*94
3A= 3/1*4+3/4*7+3/7*10+....+3/91*94
3A=1/1-1/4+1/4-1/7+1/7-1/10+............+1/91-1/94
3A=1-1/94=93/94=>A=93/94*1/3=31/94
=31/94 k mình nha bạn
1/1x4+1/4x7+1/7x10+..+1/91x97
=1-1/4+1/4-1/7+1/7-1/10+...-1/91+1/97
=1-1/97
=96/97