tính nhanh nếu có thể
a 168 x 168 - 168 x 58 / 110
b 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110 + 1/132 + 1/156
c 1/1/3 x 1/1/8 x 1/1/15 x 1/1/24 x 1/135 x ... x 1/1/99
cau C là hỗn số nha các bạn
Tìm x biết:
1/30+1/42+1/56+1/72+1/90+1/110+1/132-x=2/3
Các bạn giúp minh với nhé.
tìm x biết 1/30 + 1/42+1/56+1/72+1/90+1/110+1/132 - x=2/3
Bài 3 : Tính nhanh :
a ) A = 2 + 4 + 6 + 8 + ....+ 1000
b ) 1/ 20 + 1/30 + 1/42 + 1/50 + 1/72 + 1/90 + 1/110 + 1/132
c ) 2 / 1x3 + 2 / 3x5 + 2/5 x 7 + .....+ 2 / 97 x 99 + 2/99 x 101
a) A = 2 + 4 + 6 + 8 + ... + 1000
Ta có : A = 2 + 4 + 6 + 8 + ... + 1000 ( có 500 số )
= (1000 + 2) . 500 : 2 = 250500
c) \(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{97.99}+\frac{2}{99.101}\)
\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{97}-\frac{1}{99}+\frac{1}{99}-\frac{1}{101}\)
\(=1-\frac{1}{101}=\frac{100}{101}\)
Tìm x biết : 1/30 + 1/42 +1/56 +1/72 + 1/90 + 1/110 + 1/132 - x = 2/3
Tìm x biết 1/30+1/42+1/56+1/72+1/90+1/110+1/132 -x = 2/3
Ta có:
1/30+1/42+1/56+1/72+1/90+1/110+1/132 -x = 2/3
=>1/(5.6) + 1/(6.7) + 1/(7.8) + (1/8.9) + 1/(9.10) + 1/(10.11) + 1/(11.12) - x = 2/3
=>1/5-1/6+1/6-1/7+...+1/11-1/12 - x = 2/3
=>1/5-1/12 - x = 2/3
=>7/60 - x = 2/3
=> x = 7/60 - 2/3
=> x = -11/20
Ta có:
1/30+1/42+1/56+1/72+1/90+1/110+1/132 -x = 2/3
=>1/(5.6) + 1/(6.7) + 1/(7.8) + (1/8.9) + 1/(9.10) + 1/(10.11) + 1/(11.12) - x = 2/3
=>1/5-1/6+1/6-1/7+...+1/11-1/12 - x = 2/3
=>1/5-1/12 - x = 2/3
=>7/60 - x = 2/3
=> x = 7/60 - 2/3
=> x = -11/20
1/30+1/42+1/56+1/72+1/90+1/110+1/132=21/x
\(\Rightarrow\dfrac{1}{5\times6}+\dfrac{1}{6\times7}+\dfrac{1}{7\times8}+...+\dfrac{1}{11\times12}=\dfrac{21}{x}\\ \Rightarrow\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+...+\dfrac{1}{11}-\dfrac{1}{12}=\dfrac{21}{x}\\ \Rightarrow\dfrac{1}{5}-\dfrac{1}{12}=\dfrac{21}{x}\\ \Rightarrow\dfrac{21}{x}=\dfrac{7}{60}\Rightarrow x=\dfrac{21\cdot60}{7}=180\)
\(\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}+\dfrac{1}{110}+\dfrac{1}{132}=\dfrac{21}{x}\)
\(\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9}+\dfrac{1}{9.10}+\dfrac{1}{10.11}+\dfrac{1}{11.12}=\dfrac{21}{x}\)
\(\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{12}=\dfrac{21}{x}\)
\(\dfrac{1}{5}-\dfrac{1}{12}=\dfrac{21}{x}\)
Còn lại bạn tự tính
b)Tính thuận tiện:8,12 x 6 + 16,24 x 4 -4,06 x 8
A)tính nhanh:1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110
c)tìm a biết : 20% x a + 0,4 x a =12
giúp mk nhé ,mk cần gấp
b: =8,12(6+8-4)=8,12x10=81,2
a: \(A=\dfrac{1}{4\cdot5}+\dfrac{1}{5\cdot6}+...+\dfrac{1}{10\cdot11}\)
\(=\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+...+\dfrac{1}{10}-\dfrac{1}{11}\)
=1/4-1/11=7/44
c: =>0,2a+0,4a=12
=>0,6a=12
hay a=20
AI GIÚP MK BÀI NÀY VỚI. THANKS NHIỀU VÀ K CHO NẾU NHANH VÀ RÕ RANG NHÉ.!
TÌM X BIẾT: \(|\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}+\frac{1}{110}+\frac{1}{132}-x|=\frac{2}{3}\)2/3
mình biến đởi phần trong |......| rồi bạn thay vào nha
1/30 + 1/42 + 1/56 + 1/72 +1/ 90 + 1/110 + 1/132
=1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/9.10 +1/ 10.11
=1/5 -1/6 +1/6 - 1/7 +......+1/10 - 1/11
=1/5 - 1/11=11/55 - 5/55 =6/ 55
thay vào |....|=> |6/55 - x | = 2/3 => mở ra 2 trường hợp mà tính nha
chúc hok tốt
=>(1/5.6+1/6.7+1/7.8+1/9.10+1/10.11+1/11.12)-x=2/3
=>(1/5-1/+1/6-1/7+...+1/11-1/12)-x=2/3
=>(1/5-1/12)-x=2/3
=>7/60-x=2/3
=>x=7/60-2/3
=>x=-11/20
| 1/5.6 + 1/6.7+1/7.8+1/8.9+1/9.10+1/10.11+1/11.12 - X| = 2/3
| 1/5.1/6+1/6.1/7+1/7.1/8+1/8.1/9+1/9.1/10+1/10.1/11+1/11.1/12 - X| = 2/3
| 1/5.1/12 - X| = 2/3
| 1/60 - X | =2/3
-X=2/3-1/60
-X=13/20 =>> VÌ X NẰM TRONG GIÁ TRỊ TUYỆT ĐỐI NÊN X= 13/20 VÀ X= -13/20
( CÓ THỂ LÀ SỄ SAI BỞI MÌNH TỚ LƯỢC BỎ 1 SỐ BƯỚC CHO BỚT DÀI DÒNG)
Tìm x khi:
1/3 - 1/12 - 1/20 - 1/30 - 1/42 - 1/56 - 1/72 - 1/90 - 1/110 = x - 5/13
Giải:
\(\dfrac{1}{3}-\dfrac{1}{12}-\dfrac{1}{20}-...-\dfrac{1}{90}-\dfrac{1}{110}=x-\dfrac{5}{13}\)
\(\Leftrightarrow\dfrac{1}{1.3}-\dfrac{1}{3.4}-\dfrac{1}{4.5}-...-\dfrac{1}{9.10}-\dfrac{1}{10.11}=x-\dfrac{5}{13}\)
\(\Leftrightarrow\dfrac{-1}{2}\left(\dfrac{1}{1}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{9}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{11}\right)=x-\dfrac{5}{13}\)
\(\Leftrightarrow\dfrac{-1}{2}\left(\dfrac{1}{1}-\dfrac{1}{11}\right)=x-\dfrac{5}{13}\)
\(\Leftrightarrow\dfrac{-1}{2}.\dfrac{10}{11}=x-\dfrac{5}{13}\)
\(\Leftrightarrow\dfrac{-5}{11}=x-\dfrac{5}{13}\)
\(\Leftrightarrow\dfrac{-5}{11}+\dfrac{5}{13}=x\)
\(\Leftrightarrow x=\dfrac{-10}{143}\)
Vậy ...
Ta có:
\(\dfrac{1}{3}-\dfrac{1}{12}-\dfrac{1}{20}-\dfrac{1}{30}-...-\dfrac{1}{110}=x-\dfrac{5}{13}\\ -\dfrac{1}{12}-\dfrac{1}{20}-\dfrac{1}{30}-...-\dfrac{1}{110}=x-\dfrac{5}{13}-\dfrac{1}{3}\\ -\dfrac{1}{12}-\dfrac{1}{20}-\dfrac{1}{30}-...-\dfrac{1}{110}=x-\dfrac{28}{39}\\ -\left(\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+...+\dfrac{1}{110}\right)=x-\dfrac{28}{39}\\ -\left(\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}+...+\dfrac{1}{10.11}\right)=x-\dfrac{28}{39}\\ -\left(\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{10}-\dfrac{1}{11}\right)=x-\dfrac{28}{39}\\ -\left(\dfrac{1}{3}-\dfrac{1}{11}\right)=x-\dfrac{28}{39}\\ -\dfrac{8}{33}=x-\dfrac{28}{39}\\ x=-\dfrac{8}{33}+\dfrac{28}{39}\\ x=\dfrac{68}{143}\)
Vậy \(x=\dfrac{68}{143}\)