Problem 1 :
Suppose f(x) = x2 - 1, with the domain of f being the set of positive numbers.
(a) Evaluate f−1(8)
(b) Evaluate [f(8)]−1
Problem 2 :
Suppose f(x) = 4x + 6, evaluate f−1(5).
Problem 3 :
Suppose f(x) = 7x - 5, evaluate f−1(-3).
Problem 4 :
Suppose f(x) = 2 + (x - 5)/(x + 6), with the domain of f being the set of positive numbers.
(a) Evaluate f −1(4)
(b) Evaluate [f(4)]−1
Problem 5 :
Suppose h(x) = 3x2 - 4, where the domain of h is the set of positive numbers, find h−1(x).
1. Answer :
(a) Evaluate f−1(8) :
f(x) = x2 - 1
Replace f(x) by y.
y = x2 - 1
Interchange x and y.
x = y2 - 1
Solve for y.
x + 1 = y2
Taking square on both sides,
y = ±√(x + 1)
Replace y by f-1(x).
f−1(x) = ±√(x + 1)
f−1(8) = ±√(8 + 1)
= ±3
(b) Evaluate [f(8)]−1
[f(8)]-1 = 1/f(8)
= 1/(82 - 1)
= 1/(64 - 1)
= 1/63
2. Answer :
f(x) = 4x + 6
Replace f(x) by y.
y = 4x + 6
Interchange x and y.
x = 4y + 6
Solve for y.
x - 6 = 4y
y = (x - 6)/4
Replace y by f-1(x).
f-1(x) = (x - 6)/4
f−1(5) = (5 - 6)/4
= -1/4
3. Answer :
f(x) = 7x - 5
Replace f(x) by y.
y = 7x - 5
Interchange x and y.
x = 7y - 5
Solve for y.
x + 5 = 7y
y = (x + 5)/7
Replace y by f-1(x).
f-1(x) = (x + 5)/7
f−1(-3) = (-3 + 5)/7
= 2/7
4. Answer :
f(x) = 2 + (x - 5)/(x + 6)
f(x) = [2(x + 6) + (x - 5)]/(x + 6)
f(x) = [2x + 12 + x - 5]/(x + 6)
f(x) = [3x + 7]/(x + 6)
f(x) = (3x + 7)/(x + 6)
(a) Evaluate f −1(4) :
f(x) = (3x + 7)/(x + 6)
Replace f(x) by y.
y = (3x + 7)/(x + 6)
Interchange x and y.
x = (3y + 7)/(y + 6)
Solve for y.
x(y + 6) = 3y + 7
xy + 6x = 3y + 7
xy - 3y = 7 - 6x
y(x - 3) = 7 - 6x
y = (7 - 6x)/(x - 3)
Replace y by f-1(x).
f-1(x) = (7 - 6x)/(x - 3)
f−1(4) = (7 - 24)/(4 - 3)
= -17/1
= -17
(b) Evaluate [f(4)]−1.
[f(x)]−1 = 1/f(x)
[f(x)]−1 = (x + 6)/(3x + 7)
[f(4)]−1 = (4 + 6)/(12 + 7)
= 10/19
5. Answer :
h(x) = 3x2 - 4
Replace h(x) by y.
y = 3x2 - 4
Interchange x and y.
x = 3y2 - 4
Solve for y.
x + 4 = 3y2
(x + 4)/3 = y2
Taking square on both sides,
y = ±√[(x + 4)/3]
Replace y by h-1(x).
h−1(x) = ±√[(x + 4)/3]
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