I think in second solution test for cubic polynomial answer is wrong. I cheked my solution with Riemann Calculator Online (https://calculator-online.net/riemann-sum-calculator/) and answer is -4.291875000000000 (how i expected) against -4.441875 of author's test.
JavaScript Solution:
I think in second solution test for cubic polynomial answer is wrong. I cheked my solution with Riemann Calculator Online (https://calculator-online.net/riemann-sum-calculator/) and answer is -4.291875000000000 (how i expected) against -4.441875 of author's test.