Question

Tính hợp lí

B\(=\frac{1}{35}+\frac{1}{63}+\frac{1}{99}+....+\frac{1}{195}\)

Answer 1

1/5.7+1/7.9+1/99.11+...+1/13.15

=1/2(2/5.7+2/7.9+...+2/13.15)

=1/2.(1/2-1/15)

=1/2.13/30

=13/60

Answer 2

ta có :

B = 1 / 5 x 7 + 1 / 7 x 9 + 1 / 9 x 11 + ... + 1 /  13 x 15

2 x B = 2 / 5 x 7 + 2 / 7 x 9 + 2 / 9 x11 + ... + 2 / 13 x 15

2 x B = 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 + ... + 1 /13 - 1/15

2 x B = 1/5 - 1/15

2 x B = 3 / 15 - 1/15

2 x B = 2/15

      B = 2 / 15 : 2 

      B = 1/15

vậy B = 1/15

Answer 3

B=1/5.7 +1/7.9 +1/9.11+...+1/13.15

= 1/2 .( 2/5.7+2/7.9+....+2/13.15)

=1/2 . ( 1/5-1/7+1/7-1/9+...+1/13-1/15)

=1/2. (1/5-1/15)

=1/2. 2/15 = 1/15

nha ^^

Answer 4

mk chắc chắn nhé , bạn cứ làm đi