Question

[Huỳnh Minh Nghi]

Tính : A = 1/1*2  + 1/2*3 + 1/3*4 + ........ 1/18*19 + 1/19*20

Trả lời : A = ..............

Answer 1

[Đỗ Ngọc Hải]

\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{19.20}=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{19}-\frac{1}{20}=1-\frac{1}{20}=\frac{19}{20}\)

Answer 2

[nguyễn thị thu hiền]

=1/2[1/1*2 - 1/2*3 + 1/2*3 - 1/3*4 + 1/3*4 - 1/4*5 + ... + 1/18*19 - 1/19*20] 

=1/2[1/2 - 1/19*20] 

=1/2*189/380 

=189/760