Math 1270, Homework 4
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MATn 1270, HoMEwoRK 4
(1) (a) Show that any separable ODE, of the form M(北) + N(n)n\ = 0, is also exact. (b) For the first-order linear ODE n\ + d(北) n = b(北), written in the form
(d(北)n / b(北)) + n\ = 0
(ie. in the form M(北( n) + N(北( n)n\ = 0), describe an integrating factor d(北) in terms of d and b, with the property that multiplying by d yields an exact equation; ie. such that (dM)y = (dN)北 .
(c) For the ODE M(北( n) + N(北( n)n\ = 0, show that if
N北 / My
depends only on n, then multiplying by the integrating factor d(n) = ?。Q(y) dy yields an exact equation; ie. show that for this d,
e e
en e北
(2) Find the general solution of each of the following first-order ODE in implicit form, either because the equation is exact or by finding an integrating factor.
(a) (3北2n + 2北n + n ) + (3北2 + n )n2\ = 0
(b) (3北2 / 2北n + 2) + (6n /2 北2 + 3)n\ = 0
(c) (北/n / sin n)n\ = /1
(3) For fixed 9, D 想 0, find the general solution to
n\ = 9n / Dn3 (
(a) first by treating the equation as separable and using partial fractions; and
(b) second, by using Leibniz’s substitution (as in class) to change this into a first-order linear equation in a new dependent variable Q .
(c) Show that the two general solutions are identical.
(4) Solve the initial value problem below, for n(4), by making the substitution Q = n\ . n\n\\ = 2( n(0) = 1( n\ (0) = /2
(5) Find the general solution of the ODE below, by making the substitution Q = n\ and treating n as the independent variable.
n\\ + (n\ )2 = 2?一y
[You can solve the equation resulting from the substitution either as exact with an integrating factor or as a Bernoulli equation.]
(6) A body is fired straight up with its velocity equal to the escape velocity ^26H. Find: (a) The velocity of the body as a function of its distance 义 from the center of the earth. (b) How large 义 must be before the velocity is Q0 /100.
2022-10-02