Giải 

Khoảng cách là :

      2,98 - 1,99  =0,99 

Số số hạng là :

    (99,01 -1,99) : 0,99 +1 = 99

Tổng dãy số đó là :

      ( 99,01 + 1,99 ) x 99 : 2 = 4999,5

                                         Đáp số : 4999,5

k mình nha

S = (50  -49)(50  +49) + (50 - 48)(50 + 48) + .... + (50 + 48)(50  -48)

S = 50² - 49² + 50² - 48² + ..... + 50² - 48² + 50² - 49

S = 99  . 50² - 2.(0² + 1²  +.... + 49²)

Nguyễn Ngọc Quý đúng r đóTrần Hà Mi

A = 1.2 + 2.3 + 3.4 +..... + 99.100

=> 3A = 1.2.3 + 2.3.3 + 3.4.3 + … + 99.100.3 

=> 3A =  1.2.(3-0) + 2.3.(4 - 1) + 3.4.(5 - 2) + … + 99.100. (101 - 98) 

=> 3A = 1.2.3 +  2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + … +99.100.101-98.99.100

=> 3A = 98.99.100

=> A = 99.100.101/3

=> A = 33.100.101 = 333300

không biết

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

1.99+2.98+3.97+4.96+...+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+4.99+...+98.99+99.99)-(1.2+2.3+3.4+...+97.98+98.99)

=(1+2+3+4+...+98+99).99-(98.99.100)/3

={(99-1+1)/2}.100.99-(98.99.100)/3

=49,5.100.99-(98.99.100)/3

=4950.99-(98.99.100)/3

=4950.3.33-98.100.33

B=14850.33-9800.33

B=(14850-9800).33

B=5050.33

B=166650

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

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

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

\(=99\left(1+2+3+...+99\right)-\left(2+2.3+3.4+...+98.99\right)\)

\(=\frac{99\left(1+99\right).99}{2}-\frac{98.99.100}{3}\)

\(=99.50.99-98.33.100\)

\(=499050-323400\)

\(=166650\)

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

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

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