Phúc nè tui hỏi chơi thui đc ko? Nhưng mà tui Cần nó để làm một số chuyện

a'15.16.17-14.15.16+16.17.18-....-28.29.30

=>4A=28.29.30-14.15.16=21000=>a=7000

bc,làm tương tự

a.(a+1)=a(a+1)(a+2)---(a-1)a(a+1)

5/15.16+5/16.17+5/17.18+5/18.19+5/19.20

=5(1/15-1/16+1/16-1/17+..+1/19-1/20) 

=5.(1/15-1/20)

=5.1/60

=1/12

\(\frac{5}{15.16}+\frac{5}{16.17}+\frac{5}{17.18}+\frac{5}{18.19}+\frac{5}{19.20}\)

= \(5.\left(\frac{1}{15.16}+\frac{1}{16.17}+\frac{1}{17.18}+\frac{1}{18.19}+\frac{1}{19.20}\right)\)

= \(5.\left(\frac{1}{15}-\frac{1}{16}+\frac{1}{16}-\frac{1}{17}+\frac{1}{17}-\frac{1}{18}+\frac{1}{18}-\frac{1}{19}+\frac{1}{19}-\frac{1}{20}\right)\)

= \(5.\left(\frac{1}{15}-\frac{1}{20}\right)\)

= \(5.\frac{1}{60}\)

= \(\frac{1}{12}\)

\(\frac{5}{15.16}+\frac{5}{16.17}+\frac{5}{17.18}+\frac{5}{18.19}+\frac{5}{19.20}\)

\(=5\left(\frac{1}{15.16}+\frac{1}{16.17}+...+\frac{1}{19.20}\right)\)

\(=5\left(\frac{1}{15}-\frac{1}{16}+\frac{1}{16}-\frac{1}{17}+...+\frac{1}{19}-\frac{1}{20}\right)\)

\(=5\left(\frac{1}{15}-\frac{1}{20}\right)\)

\(=5.\frac{1}{60}\)

\(=\frac{1}{12}\)

Ta có: \(\dfrac{16.17-5}{16.16+11}\)= \(\dfrac{16.\left(16+1\right)-5}{16.16+11}\)

= \(\dfrac{16.16+16-5}{16.16+11}\)= \(\dfrac{16.16+11}{16.16+11}\)= 1

\(\dfrac{16.17-5}{16.16+11}\) = \(\dfrac{16.\left(16+1\right)-5}{16.16+11}\) = \(\dfrac{16.16+16-5}{16.16+11}\) =\(\dfrac{16.16+11}{16.16+11}=1\)

Đặt A = 2.3 + 4.5 + 6.7 + ... + 18.19

= 2(1.3 + 2.5 + ... + 9.19)

= 2(1 + 2 + 2² + 3 + 3³ + 4 + ... + 9² + 10)

= 2[(2 + 3 + 4 + ... + 10) + (1² + 2² + 3² + ... + 9²)

= \(2\left(\frac{\left(10-1\right)\left(10+2\right)}{2}+\frac{9.10.19}{6}\right)\)

= 678

Từ đó 

=> 1+2.3+4.5+..........+16.17+18.19+20

= 678 + 1 + 20 = 699

các k mk nha . mk k lại cho .

ket qua la 699 

\(\frac{\left(456.11+912\right).37}{13.74}\)

\(=\frac{\left(456.11+456.2\right).37}{13.37.2}\)

\(=\frac{[456.\left(11+2\right).37}{13.37.2}\)

\(=\frac{456.13}{13.2}\)

\(=\frac{456}{2}=228\)