Question

1X3/1+2X4/1+3X5/1+.+18X20/1=?

Answer 1

1X3/1+2X4/1+3X5/1+.+18X20/1

=1/2*(1-1/3+1/3-1/5+.+1/17-1/19)+1/2*(1/2-1/4+1/4-1/6+.+1/18-1/20)

=1/2*(1-1/19)+1/2*(1/2-1/20)

=1/2*18/19+1/2*9/20

=9/19+9/40

=531/760