Math 142B Homework Assignment 1
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Math 142B Homework Assignment 1
Due 11:00pm Wednesday, January 18, 2023
1. Let f(x) = x2 sin ( ) for x 0 and f(0) = 0.
(a) Show that f is differentiable at each x 0. (Use without proof the fact that sin(x) is differentiable and sin\ (x) = cos(x).)
(b) Use the definition of derivative to show that f is differentiable at x = 0 and that f\ (0) = 0.
(c) Show that f\ is not continuous at x = 0.
2. Let f(x) = x2 sin ( ) for x 0, f(0) = 0, and g(x) = x for x ∈ R. (a) Calculate f‘) for n = ±1, ±2, ±3, . . .
g (f (x)) − g (f (0))
x→0 f(x) − f(0)
3. Let f(x) =
(a) Show that f is continuous at x = 0.
(b) Show that f is discontinuous at all x 0.
(c) Show that f is differentiable at x = 0. Note that the formula f\ (x) = 2x does not apply to this function f .
4. Suppose f is differentiable at x = x0 .
f(x0 + h) − f(x0 )
h→0 h
f(x0 + h) − f(x0 − h)
h→0 2h
5. Suppose that the function f : (0, ∞) → R is differentiable and let c > 0. Define g : (0, ∞) → R by g(x) = f(cx). Using only the definition of derivative (without appealing to the chain rule), show that g\ (x) = cf\ (cx) for x > 0.
6. Let g be a function that is differentiable on an open interval I containing x0 . Define h(x) = {
(a) Show that h is continuous on I .
(b) Show that if g\ (x0 ) > 0, then there is a δ > 0 such that > 0 for 0 < |x−x0 | < δ .
7. Show that |cos(x) − cos(y)| ≤ |x − y| for all x,y ∈ R.
8. Let f be a function defined on R with the property that |f(x) − f(y)| ≤ (x − y)2 . Show that f is a constant function.
9. Let f and g be differentiable functions defined on an open interval I . Suppose a < b ∈ I with f(a) = f(b) = 0. Show that f\ (x) + f(x)g\ (x) = 0 for some x ∈ (a,b). [Hint: Consider h(x) = f(x)eg(x) .]
10. Show that ex ≤ ex for all x ∈ R.
2023-01-14