Tìm X:
a,X-20/11x13 - 20/13x15 - 20/15x17-...-20/53x55 = 3/11
b,1/21 + 1/28 + 1/36 + .....+ 2/Xx(X+1) = 2/9
x-20\11x13-20\13x15-20\15x17...-20\53x55=3\11
X -20/11x13 -20/13x15 -20/15x17 -.......-20/53x55 =3/11
\(x-\frac{20}{11.13}-\frac{20}{13.15}-\frac{20}{15.17}-...-\frac{20}{53.55}=\frac{3}{11}\)
\(\Rightarrow x-\left(\frac{20}{11.13}+\frac{20}{13.15}+\frac{20}{15.17}+...+\frac{20}{53.55}\right)=\frac{3}{11}\)
Đặt S = \(\frac{20}{11.13}+\frac{20}{13.15}+\frac{20}{15.17}+....+\frac{20}{53.55}\)
\(=10\left(\frac{2}{11.13}+\frac{2}{13.15}+\frac{2}{15.17}+...+\frac{2}{53.55}\right)\)
\(=10\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+\frac{1}{15}-\frac{1}{17}+...+\frac{1}{53}-\frac{1}{55}\right)\)
\(=10\left(\frac{1}{11}-\frac{1}{55}\right)\)
\(=10\cdot\frac{4}{55}=\frac{8}{11}\)
Thay A vào đề bài,ta được:
\(x-\frac{8}{11}=\frac{3}{11}\)
\(x=\frac{3}{11}+\frac{8}{11}\)
\(x=1\)
tìm x biết: x-20/11x13-20x13x15-20/15x17-............-20/53x55=3/1
Ta có :
\(x-\frac{20}{11.13}-\frac{20}{13.15}-\frac{20}{15.17}-...-\frac{20}{53.55}=\frac{3}{1}\)
\(x-\left(\frac{20}{11.13}+\frac{20}{13.15}+\frac{20}{15.17}+...+\frac{20}{53.55}\right)=\frac{3}{1}\)
Đặt \(A=\frac{20}{11.13}+\frac{20}{13.15}+\frac{20}{15.17}+...+\frac{20}{53.55}\)
\(A=10\left(\frac{2}{11.13}+\frac{2}{13.15}+\frac{2}{15.17}+...+\frac{2}{53.55}\right)\)
\(A=10\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+\frac{1}{15}-\frac{1}{17}+...+\frac{1}{53}-\frac{1}{55}\right)\)
\(A=10\left(\frac{1}{11}-\frac{1}{55}\right)\)
\(A=10.\frac{4}{55}=\frac{40}{55}=\frac{8}{11}\)
\(\Rightarrow x-\frac{8}{11}=\frac{3}{1}=3\)
\(x=3+\frac{8}{11}=\frac{33}{11}+\frac{8}{11}=\frac{41}{11}\)
Ủng hộ mk nha !!! ^_^
><in lỗi bn, 3/11 chứ k phải \(\frac{3}{1}\)
Ta có \(x-\frac{20}{11.13}-\frac{20}{13.15}-\frac{20}{15.17}-...-\frac{20}{53.55}=\frac{3}{11}\)
\(x-\left(\frac{20}{11.13}+\frac{20}{13.15}+\frac{20}{15.17}+...+\frac{20}{53.55}\right)=\frac{3}{11}\)
\(\text{Đ}\text{ặt}:A=\frac{20}{11.13}+\frac{20}{13.15}+\frac{20}{15.17}+...+\frac{20}{53.55}\)
\(A=\frac{20}{11.13}+\frac{20}{13.15}+\frac{20}{15.17}+...+\frac{20}{53.55}\)
\(A=10\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+\frac{1}{15}-\frac{1}{17}+...+\frac{1}{53}-\frac{1}{55}\right)\)
\(A=10\left(\frac{1}{11}-\frac{1}{55}\right)\)
\(A=10\left(\frac{1}{11}-\frac{1}{55}\right)\)
\(\Rightarrow x-\frac{8}{11}=\frac{3}{11}\)
\(=>x=1\)
Bài 1 Tìm x biết
a) (x+1/2)+(x+1/4)+(x+1/8)+(x+1/16)=1
b) x- 20/11x13-20/13x15 -...-20/53x55=3/11
BÀi 2 tìm x biết
h)2/3 x + 50% + x = 1/10
* Chú ý Dấu gạch chéo là dấu gạch ngang trong phân số nhé!
Bài 3: tìm số x biết
A) ( x+2) + ( x + 4 ) + ( x + 6) +...+ ( x + 100) = 6000
b) 1+ 2 + 3 +4 + ...+ x =15
c ) x - ( 2/ 11x13 + 20/13x15 + 20/15x17.... +20/53x55) =3/11
D) 7/4 x . ( 33/12+ 333/2020+333333/303030+33333333/42424242) = 22
e) 1/3+1/6+ 1/10+ 1/15+.....+ 2/x (x-1) = 2007/2009
g) ( 1/4+1/8+1/16+1/32+1/64+1/128+1/256). x =1
Chú ý : Dấu chấm trong câu d,g là dấu nhân nhé
Bài 3 :
b) Ta có 1+ 2 + 3 +4 + ...+ x =15
Nên \(\frac{x\left(x+1\right)}{2}=15\)
\(x\left(x+1\right)=30\)
=> \(x\left(x+1\right)=5.6\)
=> x = 5
Bài 2:
h; \(\dfrac{2}{3}\)\(x\) + 50% + \(x\) = \(\dfrac{1}{10}\)
\(\dfrac{2}{3}\)\(x\) + \(\dfrac{1}{2}\) + \(x\) = \(\dfrac{1}{10}\)
(\(\dfrac{2}{3}\)\(x\) + \(x\)) + \(\dfrac{1}{2}\) = \(\dfrac{1}{10}\)
\(x\) \(\times\) (\(\dfrac{2}{3}\) + 1) + \(\dfrac{1}{2}\) = \(\dfrac{1}{10}\)
\(x\) \(\times\) \(\dfrac{5}{3}\) + \(\dfrac{1}{2}\) = \(\dfrac{1}{10}\)
\(x\) \(\times\) \(\dfrac{5}{3}\) = \(\dfrac{1}{10}\) - \(\dfrac{1}{2}\)
\(x\) \(\times\) \(\dfrac{5}{3}\) = \(\dfrac{-2}{5}\)
\(x\) = \(\dfrac{-2}{5}\): \(\dfrac{5}{3}\)
\(x\) = - \(\dfrac{6}{25}\)
Lớp 5 chưa học số âm em nhé.
Bài 3:
a; (\(x\) + 2) + (\(x\) + 4) + (\(x+6\)) + ... + (\(x+100\)) = 6000
\(x\) + 2 + \(x\) + 4 + ... + \(x\) + 2 + 4 + 6 + ... + 100 = 6000
(\(x\) + \(x\) + \(x\) + ... + \(x\)) + (2 + 4 + ... + 100) = 6000
Xét dãy số 2; 4; ...;100;
Đây là dãy số cách đều với khoảng cách là: 4 - 2 = 2
Số số hạng của dãy số trên là: (100 - 2) : 2 + 1 = 50 (số)
Theo bài ra ta có:
\(x\) \(\times\) 50 + (100 + 2) \(\times\) 50 : 2 = 6000
\(x\) \(\times\) 50 + 102 x 50 : 2 = 6000
\(x\) \(\times\) 50 + (102 : 2) x 50 = 6000
\(x\) x 50 + 51 x 50 = 6000
\(x\) \(\times\) 50 + 2550 = 6000
\(x\) x 50 = 6000 - 2550
\(x\) x 50 = 3450
\(x\) x 50 = 3450
\(x\) = 3450 : 50
\(x\) = 69
Tim X
a,X - 20/11 x 13 - 20/13 x 15 - 20/15 x 17 -...-20/53 x 55 = 3/11
b,1/21 + 1/28 +1/36 + ... +2/Xx(x + 1) = 2/9
Ta có : A=20/11×13 + 20/13×15 +20/15×17+...+20/53×55
A = 10 ×( 2/11×13+2/13×15+...12/53×55)
A = 10 ×(1/11-1/13+1/13-1/15+1/15-1/17+...+1/53-1/55)
A = 10 × (1/11-1/55)
A =10 × 4/55
A = 8/11
1.Tìm X thuộc N
a. x-20/11.13-20/13.15-20/15.17-...-20/53.55=3/11
b. 1/21+1/28+1/36+...+2/x.(x+1)=2/9
Tìm x thuộc N biết
a, x- 20/11.13 - 20/13 . 15 - 20/15.17 - ... -20/53.55 - 3 /11
b, 1/21 + 1/28 + 1/36 + ... + 1/x ( x +1 ) = 2/9