MATH 215/255
Fall 2014
Comments about Homework 6
1) 6.1.12: This problem was done very poorly by most students. Very few students
received full marks. Most common mistakes included breaking up the value of F (s)
depending on whether t < 1 or t ≥ 1. Several other students just replaced f(t) entirely
by the step function and calculated that (i.e. ignored that it is t ∗ u1 (t). There seemed
to be a common misconception that somehow F (s) should be dependent on t.
2) 6.1.13: This question was done well by the majority of students. Typical mistakes
included algebraic mistakes when doing partial fractions, and mixing up terms when
doing the inverse Laplace transform. However in general students followed correct
procedure.
3) 6.2.2: Most students got this correct, mainly in the form of the alternative answer.
Several students took Laplace transforms instead or did not write in terms of Heaviside
function (instead writing it out in a case by case way).
4) 6.2.6: Generally this question was done well. Most students only lost one mark by
not taking into account the e−s term entirely or else just in front of the term yielding
the cosine.
5) 6.2.102: This question was done well by most students. The main errors were in
finding the Laplace transform of (t2 − 1) ∗ u(t − 1). A lot of students took it to be
e−s ∗ L(t2 )