3×7分之2+7×11分之2+11×15分之2+...+39×43分之2
=(1/2)×(1/3-1/7+1/7-1/11+1/11-1/15+.+1/39-1/43)
=1/2×(1/3-1/43)
=1/2×40/129
=20/129
即129分之20