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yeongil
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A friend of mine is tutoring a student in math, and they came across a problem that she couldn't answer. Here's a problem: both she and the student are Korean, and her English is okay, but not so good when it comes to specific terminology (like math). My Korean is very limited. The problem, if I understand her correctly, is below. Can anyone check my work to see if this is correct?
Given a function f such that f(a) = b and f-1(c) = d [f inverse of c is d]. Construct a 2x2 matrix A like this:
a b
c d
For each of the following functions, does A-1 always exist?
(1) y = x + 1
(2) y = log(x)
(3) y = 2x
(4) y = sqrt(x - 1)
(5) y = 1/x
Here are the inverses of the functions above:
(1) y = x - 1
(2) y = 10x
(3) y = log2(x)
(4) y = x2 + 1 (for x>=0 only)
(5) y = 1/x
A-1 exists if the determinant is not zero, ie. ad - bc <> 0.
Let's look at cases where ad - bc = 0.
Given the definitions above, are the only ways that the determinant is zero is if
(A) the ordered pairs (a, b) and (c, d) are the same, or
(B) c = ka and d = kb, for some number k?
Case (A): (a, b) = (c, d)
If the ordered pairs are the same, that means f(x) and f-1(x) must intersect at that point, right?
If f(x) and f-1(x) are not the same function, and if they intersect at all, they can only do so at points on the y = x line, correct?
Case (B): (c, d) = (ka, kb)
If c = ka and d = kb, for some number k, is there anything I can generalize here?
For functions (1) - (4), none of them and their inverses intersect (case A), and I can't find an ordered pair in the original functions so that the multiple of their coordinates satisfy the inverses functions (case B). Is it safe to say that so for those, A-1 always exist?
For function (5), f(x) = f-1(x), so any ordered pair in f(x) will also hold true in f-1(x). So for Case A, the determinant is 0, so A-1 won't exist. I can find plenty of pairs of points that would satisfy case B [like (2, 1/2) and (4, 1/4)], making the determinant 0, which means that A-1 won't exist.
So function (5) is the only one that doesn't work. Am I right? Is there anything that I'm missing? Has anyone seen a problem like this? TIA for checking.
01
Homework Statement
Given a function f such that f(a) = b and f-1(c) = d [f inverse of c is d]. Construct a 2x2 matrix A like this:
a b
c d
For each of the following functions, does A-1 always exist?
Homework Equations
(1) y = x + 1
(2) y = log(x)
(3) y = 2x
(4) y = sqrt(x - 1)
(5) y = 1/x
The Attempt at a Solution
Here are the inverses of the functions above:
(1) y = x - 1
(2) y = 10x
(3) y = log2(x)
(4) y = x2 + 1 (for x>=0 only)
(5) y = 1/x
A-1 exists if the determinant is not zero, ie. ad - bc <> 0.
Let's look at cases where ad - bc = 0.
Given the definitions above, are the only ways that the determinant is zero is if
(A) the ordered pairs (a, b) and (c, d) are the same, or
(B) c = ka and d = kb, for some number k?
Case (A): (a, b) = (c, d)
If the ordered pairs are the same, that means f(x) and f-1(x) must intersect at that point, right?
If f(x) and f-1(x) are not the same function, and if they intersect at all, they can only do so at points on the y = x line, correct?
Case (B): (c, d) = (ka, kb)
If c = ka and d = kb, for some number k, is there anything I can generalize here?
For functions (1) - (4), none of them and their inverses intersect (case A), and I can't find an ordered pair in the original functions so that the multiple of their coordinates satisfy the inverses functions (case B). Is it safe to say that so for those, A-1 always exist?
For function (5), f(x) = f-1(x), so any ordered pair in f(x) will also hold true in f-1(x). So for Case A, the determinant is 0, so A-1 won't exist. I can find plenty of pairs of points that would satisfy case B [like (2, 1/2) and (4, 1/4)], making the determinant 0, which means that A-1 won't exist.
So function (5) is the only one that doesn't work. Am I right? Is there anything that I'm missing? Has anyone seen a problem like this? TIA for checking.
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