Question

tính nhanh:

\(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+.......+\frac{3}{40.43}\)

 

Answer 1

=1/1-1/4+1/4-1/7+1/7-1/10+...+1/40-1/41

=1-1/41=40/41

Answer 2

= 3(1/1.4+1/4.7+1/7.10+.......+1/40.43)

=3(1-1/4+1/4-1/7+1/7-1/10+....+1/40-1/43)

=3(1-1/43)

=3.42/43

=126/43

Answer 3

3/1.4+3/4.7+3/7.10+.....+3/40.43

=1/1-1/4+1/4-1/7+1/7-1/10+1/10-.....+1/40-1/43

=1+(-1/4+1/4)+(-1/7+1/7)+(-1/10+1/10)+.....-1/43

=1+0+0+0+....-1/43

1-1/43=43/43-1/43=42/43