   Chapter 3.5, Problem 21E

Chapter
Section
Textbook Problem

# 1-40 Use the guidelines of this section to sketch the curve. y = ( x − 3 ) x

To determine

To sketch:

The curve of given function.

Explanation

1) Concept:

i) A domain is the set of x values that satisfy the function.

ii) To find x-intercept, put y=0, and to find y-intercept, put x=0 in the given function.

iii) Symmetry: To find symmetry, replace x by –x and check the behaviour of function. Thus, if f-x=fx, then it is an even function, so it has y-axis symmetry. If f-x=-fx, then it is an odd function, so it has x-axis symmetry. And if f-x-fxfx, then it has no symmetry.

iv) An asymptote is a tangent at infinity. To find horizontal, vertical, and slant asymptote, follow the rules.

v) A function is increasing if f'x>0  and decreasing if f'x<0 in that particular interval.

vi) The number f(c) is a local maximum value of f  if fcf(x) when x is near c and is a local minimum value of f if fc f(x) when x is near c.

vii) If f''x>0, the function is concave up and if f''x<0, the function is concave down in that particular interval. And if f''x=0, give the values of inflection points.

2) Given:

y=x-3x

3) Calculation:

Here, first find the domain of the given function and the x & y intercepts. Next, check the symmetry, asymptotes, intervals of increase and decrease, local maximum and minimum values, concavity, and points of inflection. Using these, sketch the curve.

A) Domain:

Since y=x-3x, the value of x in radicals must be  0.

Therefore, its domain is 0, .

B) Intercepts

For y intercept, plug x=0  in the given function, and solve it for y.

y=0-30

y=0

y  Intercept: (0, 0)

For x intercept, plug y=0 in the original function, and solve it for x.

0=x-3x

x=3, 0

x  Intercepts are 0, 3 and (0, 0).

C) Symmetry

For symmetry, replace x by -x.

f-x= -x-3-x

f-xfx -fx

The function is neither odd nor even.

D) Asymptote:

Horizontal asymptotes:

limx-  x-3x=-, limxx-3x=

Therefore, no horizontal asymptote.

Vertical asymptotes:

None

Slant asymptotes:

No slant asymptote.

E) Intervals of increase or decrease:

To find the intervals of increase or decrease, find the derivative of the given function.

f'x=32x-32x

Equating this derivative with 0,

32x-32x=0

Solving this for x, x=1

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