MA 3457 / CS 4033
HW #4
1. (10 points total) Circuits
In a circuit with impressed voltage ε(t) and inductance L, Kirchhoff’s first law gives the relationship
ε(t) = L
di
dt + Ri
where R is the resistance in the circuit and i is the current. Suppose we measure the current i for
several values of t and obtain:
t 1.00 1.01 1.02 1.03 1.04
i 3.01 3.12 3.14 3.18 3.24
where t is measured in seconds, i is in amperes, the inductance L is a constant 0.98 henries, and the
resistance is 0.142 ohms.
(a) (6 points) Approximate the voltage ε(t) when t = 1.00, 1.01, 1.02, 1.03, and 1.04 using the derivative
approximations derived in class and/or 4.1 in Burden & Faires textbook.
(b) (4 points) Specify the order of accuracy of each approximation and whether it is a forward,
backward, or centered approximation.
You can complete this by hand or with a Matlab code.
2. (10 points total) Approximating Derivatives
(a) (5 points) Derive a method for approximating f
000(xo) whose error term is O(h
2
), by expanding
the function f in a Taylor polynomial about xo using xo ± h, xo, and xo ± 2h.
(b) (5 points) The partial derivative fx(x, y) of f(x, y) with respect to x is obtained by holding y fixed
and differentiating with respect to x. Similar for fy when holding x fixed. Using a Taylor series
expansion of a function of two variables, determine the O(h
2
) numerical approximation formulas
and associated truncation error for fx and fy.
1
