\(\text{Đặt C = 1.2 + 2.3 + 3.4 + ..... +98.99 }\)

\(\text{ Và A = 1.98 + 2.97 + 3.96 + .... + 98.1 }\)

\(\text{Khi đó : }A=1+\left(1+2\right)+....+\left(1+2+...+98\right)\)

                      \(=\frac{1.2}{2}+\frac{2.3}{2}+\frac{3.4}{2}+....+\frac{98.99}{2}\)

                      \(=\frac{1.2+2.3+3.4+....+98.99}{2}=\frac{C}{2}\)

\(\Rightarrow B=\frac{B}{\frac{2}{B}}=\frac{1}{2}\)

B=\(\frac{98}{99}\)

B = 98/99

Đặt:

 \(M=1.2+2.3+...+98.99.\)

  \(N=1.98+2.97+...+98.1\)

     \(=1+\left(1+2\right)+...+\left(1+2+...+98\right)\)

     \(=\frac{1.2}{2}+\frac{2.3}{2}+...+\frac{98.99}{2}\)

\(=\frac{1.2+2.9+...+98.99}{2}\)\(=\frac{B}{2}\)

\(B=\frac{B}{\frac{2}{B}}=\frac{1}{2}\)

\(2\)

or\(\frac{1}{2}\)

a)\(\frac{1+\left(1+2\right)+\left(1+2+3\right)+...+\left(1+2+3+...+98\right)}{1.98+2.97+3.96+....+98.1}\)

\(=\frac{\left(1+1+....+1\right)+\left(2+2+...2\right)+....+\left(97+97\right)+98}{ }\)

\(=\frac{1.98+2.97+3.96+....+97.2+98.1}{1.98+2.97+3.96+....+98.1}=1\)

tử số của E=1 +(1+2)+(1+2+3)+.....+(1+2+3+..+98)

=1.2/2  +2.3/2 +3.4/2 +.....+98.99/2

=1.2+2.3+3.4+...+98.99/2

=>E=1/2 (đpcm)

Đặt bt đó =E

ta có tử thức của E=1+(1+2)+(1+2+3)+......+(1+2+3+...+98)

=1.2/2+2.3/2+............+98.99/2

=1.2+2.3+......+98.99/2

E=1/2

hok tốt

B=\(\frac{1.\left(100-2\right)+2.\left(100-3\right)+3.\left(100-4\right)+...+98.\left(100-99\right)}{1.2+2.3+3.4+...+98.99}\)

B=\(\frac{100.\left(1+2+3+...+98\right)-\left(1.2+2.3+3.4+...+98.99\right)}{1.2+2.3+3.4+...+98.99}\)

B=\(\frac{100.\left(1+98\right).98:2}{1.2+2.3+3.4+...+98.99}-\frac{1.2+2.3+3.4+...+98.99}{1.2+2.3+3.4+...+98.99}\)

B=\(\frac{50.98.99}{1.2+2.3+3.4+...+98.99}\)

Đặt M = 1.2+2.3+3.4+....+98.99

=> 3M=3.(1.2+2.3+3.4+...+98.99)

=> 3M = 1.2.3+2.3.(4-1)+...+098.99.(100-97)

3M= 1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+98.99.100-97.98.100

3M=98.99.100

=> M = 98.33.100

=> B = \(\frac{50.98.99}{98.33.100}-1=\frac{3}{2}-1=\frac{1}{2}\)

thanks