Hi,

I hope this is valid:

Why does

$\displaystyle \int_a^b s_n \le x_1 \le \int_a^b t_n$ and $\displaystyle \int_a^b s_n \le x_2 \le \int_a^b t_n$

multiply by -1 and add the result to the second give

$\displaystyle 0 \le x_2-x_1 \le \int_a^b t_n - \int_a^b s_n$

and not

$\displaystyle \int_a^b s_n - \int_a^b t_n \le x_2-x_1 \le \int_a^b t_n - \int_a^b s_n$

Thanks

Regards

Craig.