1-1/2+1/42+1/56+1/72+1/90+1/110
=1/2+1/(6×7)+1/(7×8)+1/(8×9)+1/(9×10)+1/(10×11)
=1/2+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10+1/10-1/11
=1/2+1/6-1/11
=19/33
1/(13×15)+1/(15×17)+1/(17×19)+...+1/(37×39)
=(1/2)(1/13-1/15+1/15-1/17+1/17-1/19+...+1/37-1/39)
=(1/2)(1/13-1/39)
=(1/2)(2/39)
=1/39