Hãy nhập câu hỏi của bạn vào đây, nếu là tài khoản VIP, bạn sẽ được ưu tiên trả lời.
Giải
13 + 23 + 33 + ..... + 213 = (1+2+3+.....+21)2
= (21.22:2)2 = 2312 = 53361
Bài 1:
\(A=\dfrac{-1}{3}+1+\dfrac{1}{3}=1\)
\(B=\dfrac{2}{15}+\dfrac{5}{9}-\dfrac{6}{9}=\dfrac{2}{15}-\dfrac{1}{9}=\dfrac{18-15}{135}=\dfrac{3}{135}=\dfrac{1}{45}\)
\(C=\dfrac{-1}{5}+\dfrac{1}{4}-\dfrac{3}{4}=\dfrac{-1}{5}-\dfrac{1}{2}=\dfrac{-7}{10}\)
Bài 2:
a: \(=\dfrac{1}{5}+\dfrac{1}{2}+\dfrac{2}{5}-\dfrac{3}{5}+\dfrac{2}{21}-\dfrac{10}{21}+\dfrac{3}{20}\)
\(=\left(\dfrac{1}{5}+\dfrac{2}{5}-\dfrac{3}{5}\right)+\left(\dfrac{2}{21}-\dfrac{10}{21}\right)+\left(\dfrac{1}{2}+\dfrac{3}{20}\right)\)
\(=\dfrac{-8}{21}+\dfrac{13}{20}=\dfrac{113}{420}\)
b: \(B=\dfrac{21}{23}-\dfrac{21}{23}+\dfrac{125}{93}-\dfrac{125}{143}=\dfrac{6250}{13299}\)
Bài 3:
\(\dfrac{7}{3}-\dfrac{1}{2}-\left(-\dfrac{3}{70}\right)=\dfrac{7}{3}-\dfrac{1}{2}+\dfrac{3}{70}=\dfrac{490}{210}-\dfrac{105}{210}+\dfrac{9}{210}=\dfrac{394}{210}=\dfrac{197}{105}\)
\(\dfrac{5}{12}-\dfrac{3}{-16}+\dfrac{3}{4}=\dfrac{5}{12}+\dfrac{3}{16}+\dfrac{3}{4}=\dfrac{20}{48}+\dfrac{9}{48}+\dfrac{36}{48}=\dfrac{65}{48}\)
Bài 4:
\(\dfrac{3}{4}-x=1\)
\(\Rightarrow-x=1-\dfrac{3}{4}\)
\(\Rightarrow x=-\dfrac{1}{4}\)
Vậy: \(x=-\dfrac{1}{4}\)
\(x+4=\dfrac{1}{5}\)
\(\Rightarrow x=\dfrac{1}{5}-4\)
\(\Rightarrow x=-\dfrac{19}{5}\)
Vậy: \(x=-\dfrac{19}{5}\)
\(x-\dfrac{1}{5}=2\)
\(\Rightarrow x=2+\dfrac{1}{5}\)
\(\Rightarrow x=\dfrac{11}{5}\)
Vậy: \(x=\dfrac{11}{5}\)
\(x+\dfrac{5}{3}=\dfrac{1}{81}\)
\(\Rightarrow x=\dfrac{1}{81}-\dfrac{5}{3}\)
\(\Rightarrow x=-\dfrac{134}{81}\)
Vậy: \(x=-\dfrac{134}{81}\)
\(A=1+2+3+...+20\)\(=\left(1+19\right)+\left(2+18\right)+...+\left(9+11\right)\)
A=1+2+3+...+20
Số số hạng là: (20-1):1+1=20
A=(20+1).20:2
A=210
B=1+3+5+...+21
Số Số hạng là : (21-1):2+1=11
B=(21+1).11:2
B=121
C=2+4+6+...+22
Số số hạng là : (22-2):2+1=11
C=(22+2).11:2
C=132
Tick cho tớ nha
a.SSH : ( 20 - 1 ) : 1 + 1 = 20
tổng : (20 + 1 ) x 20 :2 = 210
b . SSH : (21 - 1) :2 +1 = 11
tổng ; (21 + 1 ) x 11 : 2 = 121
c. SSH : ( 22 - 2 ) : 2 + 1 = 11
tổng : ( 22 + 2) x 11 : 2 = 132
\(M=\dfrac{3}{1.2}+\dfrac{3}{2.3}+\dfrac{3}{3.4}+...+\dfrac{3}{20.21}\)
\(M=3\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{20}-\dfrac{1}{21}\right)\)
\(M=3\left(1-\dfrac{1}{21}\right)\)
\(M=3.\dfrac{20}{21}=\dfrac{20}{7}\)
\(N=\dfrac{4}{3}.\dfrac{9}{8}.\dfrac{16}{15}.\dfrac{25}{24}...\dfrac{100}{99}\)
\(N=\dfrac{4.9.16.25...100}{3.8.15.24...99}\)
\(N=\dfrac{2.2.3.3.4.4.5.5...10.10}{1.3.2.4.3.5.4.6...9.11}\)
\(N=\dfrac{2.3.4.5...10}{1.2.3...9}.\dfrac{2.3.4.5...10}{3.4.5...11}\)
\(N=10.\dfrac{2}{11}=\dfrac{20}{11}\)
a) \(M=\dfrac{3}{1.2}+\dfrac{3}{2.3}+\dfrac{3}{3.4}+......+\dfrac{3}{20.21}\)
= \(3.\left(\dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+......+\dfrac{1}{20}-\dfrac{1}{21}\right)\)
= \(3.\left(\dfrac{1}{1}-\dfrac{1}{21}\right)\)
= \(3.\dfrac{20}{21}\)
= \(\dfrac{20}{7}\)
b) \(N=\dfrac{4}{3}.\dfrac{9}{8}.\dfrac{16}{15}.\dfrac{25}{24}.......\dfrac{100}{99}\)
= \(\dfrac{4.9.16.25.....100}{3.8.15.24.....99}\)
= \(\dfrac{2.2.3.3.4.4.5.5.......10.10}{1.3.2.4.3.5.4.6......9.11}\)
= \(\dfrac{\left(2.3.4.5.....10\right).\left(2.3.4.5.....10\right)}{\left(1.2.3.4......9\right).\left(3.4.5.....11\right)}\)
= \(\dfrac{10.2}{1.11}\)
= \(\dfrac{20}{11}\)
Để tính nhanh được thì cần chứng minh bài toán sau theo phương pháp quy nạp( bạn học chưa nhỉ)
\(1^3+2^3+...+n^3=\left(1+2+...+n\right)^2.\)(n là số tự nhiên)
Áp dụng
\(1^3+2^3+3^3+...+21^3=\left(1+2+...+21\right)^2=\left(\frac{21.\left(21+1\right)}{2}\right)^2=53361.\)