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13 tháng 6 2017

a bn tự lm nha mk lm đỡ bn phần b

b=1.99+1.98+3.97+...+98.2+99.1

= 1.99+2.(99-1)+3.(99-2)+...+98.(99-97)+99.(99-98)

= 1.99+2.99-1.2+3.99-2.3+...98.99-97.98+99.99-99.98

=(1.99+2.99+3.99+...+98.99+99.99)-(1.2+2.3+...+98.99)

=99.(1+2+3+...+98+99)-(1.2+2.3+...+98.99)

=99.4950-(1.2+2.3+...+98.99)

=490050-(1.2+2.3+...+98.99)

b=1.2+2.3+...+98.99

3b=1.2.3+2.3.3+...+98.99.3

3b=1.2.3+2.3.(4-1)+...+98.99.(100-97)

3b=1.2.3+2.3.4+2.3.1+...+98.99.100+98.00.97

3b=490050-(98.99.100):3

b=490050-323400

b=166650

tk mk nha

=1.99+2.(99-1)+3.(99-2)+...+98.(99-97)+99(99-98)

=99.(1+2+3+4+...+98+99)-(2+2.3+3.4+...+97.98+98.99)

=99.(1+99).99/2-98.99.100/3

=99.50.99-98.33.100

=490050-323400

=166650

13 tháng 6 2017

A= (2018+1) x 2018 : 2 = 2037171 

B= ko biết làm

31 tháng 1 2016

1.99+2.98+3.97+...+98.2+99.1=1.99+2.(99-1)+3.(99-2)+...+98.(99-97)+99.(99-98)

=1.99+2.99-1.2+3.99-2.3+...+98.99-97.98+99.99-98.99

=(1.99+2.99+3.99+...+98.99+99.99)-(1.2+2.3+3.4+...+98.99)

=99.(1+2+...+99)-(1.2+2.3+...+98.99)=99.4950-(1.2+2.3+...+98.99)=490050-(1.2+2.3+...+98.99)

đặt A=1.2+2.3+...+98.99

=>3A=1.2.3+2.3.3+...+98.99.3

=1.2.3+2.3.(4-1)+...+98.99.(100-97)

=1.2.3-1.2.3+2.3.4-2.3.4+...+97.98.99-97.98.99+98.99.100=98.99.100

=>A=98.99.100:3=323400

=>1.99+2.98+3.97+...+98.2+99.1=490050-323400=166650

31 tháng 1 2016

1,99+2,98+3,97+...+98,2+99,1 thì sẽ bằng => 166 650

13 tháng 6 2017

\(C=1.99+2.98+3.97+........+97.3+98.2+99.1\)

\(\Rightarrow C=1.99+2.\left(99-1\right)+3\left(99-2\right)+..........+98.\left(99-97\right)+99.\left(99-98\right)\)

\(\Rightarrow C=1.99+2.99-1.2+3.99-2.3+........+98.99-97.98+99.99-98.99\)

\(\Rightarrow C=\left(1.99+2.99+.......+99.99\right)-\left(1.2+2.3+.........+98.99\right)\)

\(\Rightarrow C=490050-\left(1.2+2.3+....+98.99\right)\)

Đặt \(A=1.2+2.3+3.4+........+98.99\)

\(\Rightarrow3A=1.2.3+2.3.3+..........+98.99.3\)

\(\Rightarrow3A=1.2.3+2.3.\left(4-1\right)+.....+98.99\left(100-97\right)\)

\(\Rightarrow3A=1.2.3-1.2.3+2.3.4-2.3.4+......+97.97.99-97.98.99+98.99.100\)

\(\Rightarrow3A=98.99.100\)

\(\Rightarrow A=\dfrac{98.99.100}{3}=323400\)

\(\Rightarrow C=490050-323400=166650\)

Vậy \(C=166650\)

13 tháng 6 2017

Xem lại đề đi bạn. Đề có vấn đề?!

26 tháng 9 2021

\(a,A=1\cdot2+2\cdot3+...+98\cdot99\\ 3A=1\cdot2\cdot3+2\cdot3\cdot3+3\cdot4\cdot3+...+98\cdot99\cdot3\\ 3A=1\cdot2\cdot3+2\cdot3\cdot\left(4-1\right)+3\cdot4\left(5-2\right)+...+98\cdot99\left(100-97\right)\\ 3A=1\cdot2\cdot3-1\cdot2\cdot3+2\cdot3\cdot4-2\cdot3\cdot4+3\cdot4\cdot5-...-97\cdot98\cdot99+98\cdot99\cdot100\\ 3A=98\cdot99\cdot100=970200\\ A=323400\)

\(b,B=1^2+2^2+3^3+...+98^2\\ B=1\left(2-1\right)+2\left(3-1\right)+3\left(4-1\right)+...+98\left(99-1\right)\\ B=\left(1\cdot2+2\cdot3+3\cdot4+...+98\cdot99\right)-\left(1+2+...+98\right)\\ B=323400-\left[\left(98+1\right)\left(98-1+1\right):2\right]\\ B=323400-4851=318549\\ c,C=1\cdot99+2\left(99-1\right)+3\left(99-2\right)+...+98\left(99-97\right)+99\left(99-98\right)\\ C=1\cdot99+2\cdot99-1\cdot2+3\cdot99-2\cdot3+...+98\cdot99-97\cdot98+99\cdot99-98\cdot99\\ C=99\left(1+2+...+99\right)-\left(1\cdot2+2\cdot3+...+98\cdot99\right)\\ C=99\left[\left(99+1\right)\left(99-1+1\right):2\right]-323400\\ C=490050-323400=166650\)

26 tháng 9 2021

https://hoc24.vn/cau-hoi/a-tinh-tong-a1223349899b-su-dung-ket-qua-cau-a-tinh-b122232972982c-su-dung-ket-qua-cau-a-tinh-c1992983979829.2030286199021

:vv hỏi hoài z?

10 tháng 12 2016

A bài thầy Phú

Ta có: N=1.99+2.98+3.97+..+98.2

=(99-98).99+(99-97).98+(99-96).97+..+(99-1).2

=99.99-98.99+98.99-97.98+99.97-96.97+..+99.2-1.2

=99.99+99.98+99.97+..+99.2-(99.98+98.97+97.96+..+1.2)

=99.(99+98+97+..+2)-323400 (cm a)

Xét dãy 99+98+97+..+2

SSH:(99-2):1+1=98

Tổng: (99+2).98:2=4949

=>N=99.4949-323400

=489951-323400=166551