Question

1 Tính

 

c) \(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+..................+\frac{1}{90}=\)

giúp mik nha, mik kb cho

Answer 1

1/3 + 1/6 + 1/9 + 1/18

= 6/18 + 3/18 + 2/18 + 1/18

= 12/18

= 2/3

Answer 2

b)  42/24 - 10/8 - 2/4

= 42/24 - 30/24 - 12/24

= 0

Answer 3

=1/2x3 + 1/3x4 + 1/ 4x5 +.............+1/9x10

=3-2/2x3 + 4-3/3x4 + 5-4/4x5 + ..........+ 10-9/9x10

=3/2x3 - 2/3x2 + 4/3x4 - 3/4x3 + 5/4x5 - 4/4x5 + .............+10/9x10 - 9 /10x9

=1/2-1/3+1/3-1/4+1/4-1/5+.........................+1/9-1/10

=1/2-1/10

=2/5