Question

(2/1-3/1）+（3/1-4/1）+（4/1+5/1）+......+（19/1-20/1）=？

Answer 1

(2/1-3/1)+(3/1-4/1)+(4/1-5/1)+……+(19/1-20/1)

=2/1-3/1+3/1-4/1+4/1-5/1+……+18/1-19/1+19/1-20/1

=2/1-20/1

=20/9