Câu 1:1/18+1/54+1/108+.................................+1/990
Câu 2:1/25x27+1/27x29+1/29x31+.................................+1/73x75
B=1/25x27+1/27x29+1/29x31+...+1/73x75
B bằng bao nhiêu hỏi gấp trong hôm nay
\(B=\frac{1}{25.27}+\frac{1}{27.29}+\frac{1}{29.31}+...+\frac{1}{73.75}\)
\(B=\frac{1}{2}\left(\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+\frac{1}{29}-\frac{1}{31}+...+\frac{1}{73}-\frac{1}{75}\right)\)
\(B=\frac{1}{2}\left(\frac{1}{25}-\frac{1}{75}\right)\)
\(B=\frac{1}{2}\cdot\frac{2}{75}\)
\(B=\frac{1}{75}\)
\(B=\frac{1}{25.27}+\frac{1}{27.29}+\frac{1}{29.31}+....+\frac{1}{73.75}\)
\(B=\frac{1}{2}.\left(\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+\frac{1}{29}-\frac{1}{31}+...+\frac{1}{73}-\frac{1}{75}\right)\)
\(B=\frac{1}{2}\cdot\left(\frac{1}{25}-\frac{1}{75}\right)\)
\(B=\frac{1}{2}\cdot\frac{2}{75}=\frac{1}{75}\)
Theo đề bài ta có:
\(B=\frac{1}{25.27}+\frac{1}{27.29}+\frac{1}{29.31}+...+\frac{1}{73.75}\)
\(\Leftrightarrow B=\frac{1}{2}.\left(\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+...+\frac{1}{73}-\frac{1}{75}\right)\)
\(\Leftrightarrow B=\frac{1}{2}.\left(\frac{1}{25}-\frac{1}{27}\right)\)
\(\Leftrightarrow B=\frac{1}{2}\cdot\frac{2}{75}\)
\(\Leftrightarrow B=\frac{1}{75}\)
Tìm x:
\(\left(\frac{1}{25x27}+\frac{1}{27x29}+\frac{1}{29x31}+...+\frac{1}{73x75}\right)x\frac{3}{10}=x\)
\(\left(\frac{1}{25x27}+\frac{1}{27x29}+\frac{1}{29x31}+....+\frac{1}{73x75}\right)x\frac{3}{10}=x\)
\(< =>\frac{1}{2}x\left(\frac{2}{25x27}+\frac{2}{27x29}+\frac{2}{29x31}+....+\frac{2}{73x75}\right)x\frac{3}{10}=x\)
\(< =>\frac{1}{2}x\left(\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+\frac{1}{29}-\frac{1}{31}+....+\frac{1}{73}-\frac{1}{75}\right)x\frac{3}{10}=x\)
\(< =>\frac{1}{2}x\left(\frac{1}{25}-\frac{1}{75}\right)x\frac{3}{10}=x< =>\frac{1}{2}x\frac{2}{25}x\frac{3}{10}=x< =>x=\frac{3}{250}\)
Tính nhanh :a) 4/3x7 +4/7x11 +4/11x15 +...+4/ 107x111
b )2/15 +2/35 +2/63 +...+ 2/399
c)1/25x27 +1/27x29 +1/29x31 +...+ 1/73x75
1. Tính các tổng sau:
a) D= 6/15x18 + 6/18x21 + 6/21x24 + .......... + 6/89x90
b) F= 1/25x27 + 1/27x29 + 1/29x31 + .............+1/73x75
^^^^^^^^^ Các bạn cố gắng giúp mình nha!!!!!!
D=6/15.18+6/18.21+...+6/89.92
D=6(1/15.18+1/18.21+...+1/89.92)
a) 3D=6(1/15-1/18+1/18-1/21+...+1/89-1/92)
3D=6(1/15-1/92)
3D=6.(77/1380)
3D=77/230
D=77/690
b) F=1/25.27+1/27.29+...+1/73.75
2F=2/25.27+2/27.29+..+2/73.75
2F=1/25-1/27+1/27-1/29+...+1/73-1/75
2F=1/25-1/75
2F=2/75
F=1/75
Tính tổng
A= 1/25x27 +1/27x29 +.............+1/73x75
B=15/ 90x94 + 15/94x98 +...............+15/146x150
C=10/56 + 10/140 + 10/260 +.................+10/1400
*** Bạn nào nhanh nhất mình Tick cho nha
\(A=\frac{1}{25.27}+\frac{1}{27.29}+...+\frac{1}{73.75}=\frac{1}{2}\left(\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+...+\frac{1}{73}-\frac{1}{75}\right)\)
\(A=\frac{1}{2}\left(\frac{1}{25}-\frac{1}{75}\right)\\ A=\frac{1}{75}\)
\(B=\frac{15}{90.94}+\frac{15}{94.98}+...+\frac{15}{146+150}=\frac{1}{4}\left(\frac{15}{90}-\frac{15}{94}+\frac{15}{94}-\frac{15}{98}+...+\frac{15}{146}-\frac{15}{150}\right)\)
\(B=\frac{1}{4}\left(\frac{15}{90}-\frac{15}{150}\right)=\frac{1}{60}\)
a, A= 9/8x11 + 9/11x14 + 9/ 14x 17 +....+9/ 73x75
b, B= 1/25x27 + 1/ 27x29 + 1/29 x31 +.....+ 1/ 73 x75
c, C= 15/90x94 + 15/94x98 + 15/98x102 +......+15/146x150
d, D= 10/56 + 10/140 + 10/260 +.......+ 10/1400
e, E=6/15x18 + 6/18x21 + 6/21x24 +....+ 6/87x90
Các bạn làm nhanh giúp mình mình đang cần gấp ạ!
a) A = 9/8.11 + 9/11.14 + 9/14.17 + ... + 9/73.75
A = 3.(1/8 - 1/11 + 1/11 - 1/14 + 1/14 - 1/17 + ... + 1/73 - 1/75)
A = 3.(1/8 - 1/75)
A = 3.67/600
A = 67/200
Các bài sau làm tương tự, riêng câu D thì phân tích ra
Mình chỉ làm hộ bạn câu a) thôi nhé vì đề sàn sàn giống nhau :
a) \(A=\frac{9}{8×11}+\frac{9}{11×14}+\frac{9}{14×17}+...+\frac{9}{73×75}\)
\(A=\frac{9}{8}-\frac{9}{11}+\frac{9}{11}-\frac{9}{14}+\frac{9}{14}-\frac{9}{17}+...+\frac{9}{73}-\frac{9}{75}\)
\(A=\frac{9}{8}-\frac{9}{75}\)
\(A=\frac{675}{600}-\frac{72}{600}\)
\(A=\frac{673}{600}\)
Vậy,...
Cbht
\(d,D=\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)
\(10D=\frac{1}{7\cdot8}+\frac{1}{2\cdot70}+\frac{1}{2\cdot130}+...+\frac{1}{2\cdot700}\)
\(10D=\frac{1}{7}-\frac{1}{8}+\frac{1}{2}-\frac{1}{70}+\frac{1}{2}-\frac{1}{130}+...+\frac{1}{2}-\frac{1}{700}\)
\(10D=\frac{1}{7}-\frac{1}{700}\)
\(10D=\frac{99}{700}\)
\(D=\frac{99}{700}:10\)
\(D=\frac{99}{7000}\)
a, A= 9/8x11 + 9/11x14 + 9/ 14x 17 +....+9/ 73x75
b, B= 1/25x27 + 1/ 27x29 + 1/29 x31 +.....+ 1/ 73 x75
c, C= 15/90x94 + 15/94x98 + 15/98x102 +......+15/146x150
d, D= 10/56 + 10/140 + 10/260 +.......+ 10/1400
e, E=6/15x18 + 6/18x21 + 6/21x24 +....+ 6/87x90
Các bạn làm nhanh giúp mình mình đang cần gấp ạ!
a, A= 9/8x11 + 9/11x14 + 9/ 14x 17 +....+9/ 73x75
b, B= 1/25x27 + 1/ 27x29 + 1/29 x31 +.....+ 1/ 73 x75
c, C= 15/90x94 + 15/94x98 + 15/98x102 +......+15/146x150
d, D= 10/56 + 10/140 + 10/260 +.......+ 10/1400
e, E=6/15x18 + 6/18x21 + 6/21x24 +....+ 6/87x90
Các bạn làm nhanh giúp mình mình đang cần gấp ạ!
Tính: 1/25x27 + 1/27x29+.....1/75x79
Ai giải được mình bấm
\(\frac{1}{25.27}+\frac{1}{27.29}+.......+\frac{1}{77.79}\)
\(=\frac{1}{2}.\left(\frac{2}{25.27}+\frac{2}{27.29}+....+\frac{2}{77.79}\right)\)
\(=\frac{1}{2}.\left(\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+......+\frac{1}{77}+\frac{1}{79}\right)\)
\(=\frac{1}{2}.\left(\frac{1}{25}-\frac{1}{79}\right)\)
\(=\frac{1}{2}.\frac{54}{1975}\)
\(=\frac{27}{1975}\)
\(A=\frac{1}{25.27}+\frac{1}{27.29}+...........+\frac{1}{75.79}\)
\(\Leftrightarrow2A=\frac{2}{25.27}+\frac{2}{27.29}+........+\frac{2}{75.79}\)
\(\Leftrightarrow2A=\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+.........+\frac{1}{75}-\frac{1}{79}\)
\(\Leftrightarrow2A=\frac{1}{25}-\frac{1}{79}\)
Tự tính tiếp! chúc b hk tốt