1/1×3 + 1/2×4 + 1/3×5 +...+ 1/10×12= (1/2)×(1 - 1/3) + (1/2)×(1/2 - 1/4) + ...+ (1/2)(1/10 - 1/12)= (1/2)×(1 - 1/3 + 1/2 - 1/4 + ...+ 1/10 - 1/12)= (1/2)×(1 + 1/2 - 1/11 - 1/12)= (1/2)×175/13...