Corrections
Our worked solutions are drafted by AI and are not reviewed by a licensed engineer, so they contain errors. When a reader reports one and we confirm it, it is listed here with what was wrong and what changed. If you find an error, use Report an error in this solution at the bottom of the question page.
Confirmed errors
These were found in our own spot check of 30 random questions in October 2026. The pages themselves have not been corrected yet; read them together with the note here.
23-Ind-A5 Quality Planning, Control, and Assurance, December 2019, Question 5(c) (found 2026-10-01). Wrong: for the 25−2 design with D = AB and E = AC, the solution estimates only the 5 main effects and concludes that B and C can be set at any convenient level. The design has 7 estimable contrasts. The two it leaves out are BC ≡ DE ≈ −3.75 and ABC ≡ BE ≡ CD ≈ −9.25, and the second is the largest effect, so B and C matter through a strong interaction and cannot be set freely.
18-Geol-A5 Rock Mechanics, December 2014, Question 3 (found 2026-10-01). Wrong: the solution treats the contact between blocks A and B as frictionless, although the question gives φ = 35° for all surfaces. With friction on that contact (Goodman–Bray sliding-block analysis), P ≈ 0.4175 WA and the required thickness is t ≈ 1.02 m, not 2.00 m. The conclusion that there is no slip at C still holds.
22-Elec-A2 Systems and Control, May 2017, Question 8, part 4 (found 2026-10-01). Misleading: the gains are right (Kp ≈ 14.97, Ki ≈ 5.56, Kd ≈ 0.171), but the solution calls the zero at −5.85 "far" from the dominant poles and its effect "small". It is only about 2.9 times the dominant real part, and a simulated step response overshoots about 20% against the 15% design target.