Tìm X:
a,2xX-5=21
b,3/4+1/4xX=5/6
c,7/8-1/2xX=5/6
d(1/1x2+1/2x3+1/3x4)x100-[5/2:(X+206/100)]:1/2=89
(1/1x2+1/2x3+1/3x4.....1/7x8+1/8x9)x100-[5/2-(x+206/100)]:1/2=89
Tìm X:
(1/1x1+1/2x3+1/3x4+............+1/9x10)x100-[5/2:(X+206/100)]:1/2=89
\(\left(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+...+\frac{1}{9\cdot10}\right)\cdot100-\left[\frac{5}{2}:\left(x+\frac{206}{100}\right)\right]:\frac{1}{2}=89\)
\(\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\right)\cdot100-\left[\frac{5}{2}:\left(x+\frac{103}{50}\right)\right]\cdot2=89\)
\(\left(1-\frac{1}{10}\right)\cdot100-\frac{5}{2}:\left(x+\frac{103}{50}\right)\cdot2=89\)
\(\frac{9}{10}\cdot100-\frac{5}{2}\cdot2:\left(x+\frac{103}{50}\right)=89\)
\(90-5\cdot\left(x+\frac{103}{50}\right)=89\)
\(5\cdot\left(x+\frac{103}{50}\right)=1\)
\(x+\frac{103}{50}=\frac{1}{5}\)
\(x=-\frac{93}{50}\)
(1/1x2+1/2x3+1/3x4+........+1/8x9+1/9x10)x100-(5/2:x+(206/100:1/2)
Tìm x trong biểu thức sau:
\(\left(\frac{1}{1x2}+\frac{1}{2x3}+\frac{1}{3x4}+....+\frac{1}{8x9}+\frac{1}{9x10}\right)x100-\left[\frac{5}{2}:\left(x+\frac{206}{100}\right)\right]:\frac{1}{2}=89\)
( 1/1x2 + 1/2x3 + 1/3x4 + ... + 1/9/10 ) x 100 - ( 5/2 : ( x + 206/100 ) : 1/2 = 89
Bài 1. Tính nhanh :
2/1x2 + 2/2x3 + 2/3x4 + ... + 2/18x19 +2/19x20 .
Bài 2. Tìm x trong biểu thức sau :
( 1/1x2 + 1/2x3 + 1/3x4 + ... + 1/8x9 + 1/9x10 ) x 100 - [5/2 : ( x + 206/100 )] :1/2 = 89
CÁC BẠN GIÚP MÌNH NHA !
Bài 1:
Đặt \(A=\frac{2}{1x2}+\frac{2}{2x3}+\frac{2}{3x4}+...+\frac{2}{18x19}+\frac{2}{19x20}\)
\(\frac{A}{2}=\frac{1}{1x2}+\frac{1}{2x3}+\frac{1}{3x4}+...+\frac{1}{18x19}+\frac{1}{19x20}\)
\(\frac{A}{2}=\frac{2-1}{1x2}+\frac{3-2}{2x3}+\frac{4-3}{3x4}+...+\frac{19-18}{18x19}+\frac{20-19}{19x20}\)
\(\frac{A}{2}=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{18}-\frac{1}{19}+\frac{1}{19}-\frac{1}{20}=1-\frac{1}{20}=\frac{19}{20}\)
\(A=\frac{2x19}{20}=\frac{19}{10}\)
Bài 2:
Đặt \(B=\frac{1}{1x2}+\frac{1}{2x3}+\frac{1}{3x4}+...+\frac{1}{8x9}+\frac{1}{9x10}\)
Làm tương tự câu 1 có \(B=1-\frac{1}{10}=\frac{9}{10}\)
\(Bx100=\frac{9}{10}x100=90\)
=> \(\left[\frac{5}{2}:\left(x+\frac{206}{100}\right)\right]:\frac{1}{2}=1\)
=> \(\left[\frac{5}{2}:\left(x+\frac{206}{100}\right)\right]=\frac{1}{2}\)
=> \(x+\frac{206}{100}=\frac{5}{2}:\frac{1}{2}=5\Rightarrow x=5-\frac{206}{100}=\frac{294}{100}=\frac{147}{50}\)
đáp án bài 1 là: 19/10
đáp án bài 2 là 147/50
cho dãy số x+1; 2xX+3; 3xX+5; 4xX+7;..... Viết số hạng thứ 100 và tìm x để tổng của 100 số hạng đầu tiên của dãy là 60500
A=5/9+2/15-6/9
B=2/7-3/8+4/7+1/7-5/8+5/15
C=3/5+1/15+1/57+1/3-2/9-3/4-1/36
D=1/1x2+1/2x3+1/3x4+...+1/99x100
E=1-3/1x3-3/3x5-3/5x7-...-19
help plssssssss
Giải:
A=5/9+2/15-6/9
=(5/9-6/9)+2/15
= -1/9 + 2/15
= 1/45
B=2/7-3/8+4/7+1/7-5/8+5/15
= (2/7+4/7+1/7) + (-3/8-5/8) +1/3
= 1+ (-1) +1/3
=1/3
C=3/5+1/15+1/57+1/3-2/9-3/4-1/36
=9/15+1/15+1/57+19/57-8/36-27/36-1/36
=(9/15+1/15)+(1/57+19/57)+(-8/36-27/36-1/36)
=2/3+20/57+(-1)
=58/57+(-1)
=1/57
D=1/1.2+1/2.3+1/3.4+...+1/99.100
=1/1-1/2+1/2-1/3+1/3-1/4+...+1/99-1/100
=1/1-1/100
=99/100
Câu E mình ko biết làm nhé!
Tính hợp lí nếu có thể:
a)3/7+4/9+4/7+5/9
b)1/5+4/10+9/15+16/20+25/25+36/30+49/35+64/40+81/45
c)1/8+1/12+3/8+5/12
d)(1-1/2)x(1-1/3)x(1-1/4)x...x(1-100)
e)9/5:17/15+8/5:17/15
f)2/1x2+2/2x3+2/3x4+...+2/19x20
Ai nhanh nhất mình tick cho
a) 3/7 + 4/9 + 4/7 + 5/9
= ( 3/7 + 4/7 ) + ( 4/9 + 5/9 )
= 7/7 + 9/9
= 1 + 1
= 2
b)1/5 + 4/10 + 9/15 + 16/20 + 25/25 + 36/30 + 49/35 + 64/40 + 81/45
= 1/5 + 2/5 + 3/5 + 4/5 + 5/5 + 6/5 + 7/5 + 8/5 + 9/5
= ( 1/5 + 9/5 ) + ( 2/5 + 8/5 ) + (7/5 + 3/5 ) + ( 4/5 + 6/5 ) + 5/5
= 2 + 2 + 2 + 2 + 1
= 2 x 4 + 1
= 8 +1
= 9
c) 1/8 + 1/12 + 3/8 + 5/12
= ( 1/8 + 3/8 ) + ( 1/12 + 5/12)
= 4/8 + 6/12
= 1/2 + 1/2
= 2/4 = 1/2
mỏi tay rồi