D=1/2+1/6+1/12+1/20+1/30+1/42+1/56
Cau hoi mang tinh chat do thich thi tra loi trinh bay ca cach lam nha
Bai 1:
giai bai toan sau = cach lap phuong trinh hoac he phuong trinh
cau 1:
hai to cung lam chung 1 cong viec trong 12 gio xong .Nhung 2 to cung lam 4 gio thi to 1 di lam viec khac, to 2 lam not trong 10 gio thi xong . Hoi moi to lam rieng trong bao lau thi xong cong viec?
cau 2:
hai voi nc cung chay vao be ko chua nc trong 1 gio 30 phutthi day be . neu voi 1 chay trong 15 phut roi khoa lai sau do mo voi 2 chay trong 20 phut thi dc 20% be . Hoi moi voi chay rieng trong bao lau se day be?
cau 3:
luc 7 gio mot oto di tu A den B, sau do 30 phut mot xe may di tu B ve A voi van toc kem van toc oto la 24 km/h. Oto di den B dc 1 gio 20 phut thi xe may moi den A. tinh vsn toc moi xe . Biet quang duong tu AB la 120km
Tinh nhanh:
1/6+1/12+1/20+1/30+1/42+1/56
Lam di nha ai lam nhanh thi minh tick!
Tổng quát: \(\frac{1}{n\left(n+1\right)}=\frac{1}{n}-\frac{1}{n+1}\)
Ta có: \(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}\)
\(=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)
\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\)
\(=\frac{1}{2}-\frac{1}{8}=\frac{3}{8}\)
1/6+1/12+1/20+1/30+1/42+1/56
=1/2x3+1/3x4+1/4x5+1/5x6+1/6x7+1/7x8
=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8
=1/2-1/8
=3/8
Ket qua cua tong: A = 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 = ............
Ai giai nhanh nhat va ghi ro cach lam ra ( ko ghi ....... ) thi minh se tick cho nha, minh dang can gap^^^
TA CÓ:\(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}\)
\(=\frac{1}{2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)
\(=\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\)
\(=\frac{1}{2}+\frac{1}{2}-\frac{1}{8}\)
\(=1-\frac{1}{8}=\frac{7}{8}\)
A = 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 = 7/8
= 6/12 + 2/12 + 1/12 + 3/60 + 2/60 + 8/336 + 6/336
= 9/12 + 5/60 + 14/336
= 5/6 + 1/24 = 7/8
Neu 4+2+3 =5
8+4+6 = 10
3+2+1=2
Thi 6+4+2=
Ai tra loi duoc minh se tick . Tinh ca minh do nha neu minh tra loi dc thi phai tick cho minh do,
Chuc cac ban tra loi tot nha minh se tick cho ban ao dung va nhanh nhat khong can cach giai
do vui ne! nho tra loi day du nha .tren troi co mot may bay cho 100 vien gach lam rot hot 1 vien hoi con lai bao nhieu vien gach? muon nhet con huou cao co vao tu lanh thi can co may buoc? muon nhet con voi vao tu lanh can co may buoc? tren thien dinh tat ca muon thu deu di hoi con nao khong di vi sao? mot ba gia dung o song co rat nhieu ca sau hoi vi sao ba chua qua song ma da chet truoc khi qua? nho tra loi day du va giai thich ro rang nhe!
cau dau ko biet.can 3 buoc:1 mo tu lanh ra.2:bo con huou vao.3:dong tu lanh lai./can 4 buoc:1:mo tu lanh ra.2:lay con voi ra.3:bo con voi vao.4:dong tu lai/thieu con voi vi con voi dang o trong tu lanh
1+2+3+4++5+6+7+8+9+10+11+12+13+14++15+16+17+18+19+20
Hoi tong tren bang bao nhieu (tinh nhanh va giai thich cach lam)
Ta có: 1+2+3+…+20=20.(20+1):2=20.21:2=420:2=210
khoảng cách: 2-1=1
số số hạng là: (20-1):1+1=20(số)
tổng là: (20+1)x20:2=210
Tinh :
A=\(\left(1+\frac{1}{1.3}\right)\left(1+\frac{1}{2.4}\right)\left(1+\frac{1}{3.5}\right)....\left(1+\frac{1}{2014.1016}\right)\)
Giup minh trinh bay ca cach lam nua nha!
Ai chua doc het de va ko hieu thi vao phan "Doc them" se ro
Thanks
Tinh :
A=\(\left(1+\frac{1}{1.3}\right)\left(1+\frac{1}{2.4}\right)\left(1+\frac{1}{3.5}\right)......\left(1+\frac{1}{2014.2016}\right)\)
Giup minh trinh bay ca cach lam nua nha!
Ai chua doc het de va ko hieu thi vao phan "Doc them" se ro
Thanks
Tinh gia tri cua bieu thuc \(D=\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+\frac{1}{45}\)
Nho giai va trinh bay cach lam cho minh luon nha ^_^
1/2D=1/2(1/6+1/10+......+1/45)
1/2D=1/12+1/20+1/30+.....+1/90
1/2D=1/3.4+1/4.5+1/5.6+......+1/9.10
1/2D=1/3-1/4+1/4-1/5+1/5-1/6+....+1/9-1/10
1/2D=1/3-1/10
1/2D=7/30
D=7/30:1/2
D=7/15
Ta có:\(D=\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+\frac{1}{45}\)
\(=\frac{2}{12}+\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+\frac{2}{56}+\frac{2}{72}+\frac{2}{90}\)
\(=2.\left(\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\right)\)
\(=2.\left(\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}\right)\)
\(=2.\left(\frac{1}{3}-\frac{1}{10}\right)=2.\frac{7}{30}=\frac{7}{15}\)
Vậy \(D=\frac{7}{15}\)
không biết làm