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Alex
3 years ago
11

Find dy/dx if f(x) = (x + 8)^3x.

Mathematics
1 answer:
vlabodo [156]3 years ago
8 0
This is quite a complex problem. I wrote out a really nice solution but I can't work out how to put it on the website as the app is very poorly made. Still, I'll just have to type it all in...

Okay so you need to use a technique called logarithmic differentiation. It seems quite unnatural to start with but the result is very impressive.

Let y = (x+8)^(3x)

Take the natural log of both sides:

ln(y) = ln((x+8)^(3x))

By laws of logarithms, this can be rearranged:

ln(y) = 3xln(x+8)

Next, differentiate both sides. By implicit differentiation:

d/dx(ln(y)) = 1/y dy/dx

The right hand side is harder to differentiate. Using the substitution u = 3x and v = ln(x+8):

d/dx(3xln(x+8)) = d/dx(uv)

du/dx = 3

Finding dv/dx is harder, and involves the chain rule. Let a = x+ 8:

v = ln(a)
da/dx = 1
dv/da = 1/a

By chain rule:

dv/dx = dv/da * da/dx = 1/a = 1/(x+8)

Finally, use the product rule:

d/dx(uv) = u * dv/dx + v * du/dx = 3x/(x+8) + 3ln(x+8)

This overall produces the equation:

1/y * dy/dx = 3x/(x+8) + 3ln(x+8)

We want to solve for dy/dx, achievable by multiplying both sides by y:

dy/dx = y(3x/(x+8) + 3ln(x+8))

Since we know y = (x+8)^(3x):

dy/dx = ((x+8)^(3x))(3x/(x+8) + 3ln(x+8))

Neatening this up a bit, we factorise out 3/(x+8):

dy/dx = (3(x+8)^(3x-1))(x + (x+8)ln(x+8))

Well wasn't that a marathon? It's a nightmare typing that in, I hope you can follow all the steps.

I hope this helped you :)
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A survey found that​ women's heights are normally distributed with mean 63.3 in. and standard deviation 2.7 in. The survey also
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Answer:

A) There is a 100% chance of meeting the height requirement for women.

B) There is a 99.89% chance of meeting the height requirement for men.

C) New height requirements are:

71.9 inches for men and 58.9 inches minimum for women.

At least 58.9 inches and at most 71.9 inches.

Step-by-step explanation:

This question involves finding z values which tell us the percentage from 0 to x.

z = (x-μ)/σ

A) In this question, the minimum sample point is 4 ft 9 inches or converting to inches, we have;(4*12) + 9 =57 inches.

The maximum sample point is 6 ft 4 inches which in inches gives; (6*12) + 4 = 76 inches

For the minimum point;

z = (57 - 63.3)/2.7 = -2.33

From the z-distribution table, this equates to 0.0099.

The maximum;

z = (76 - 63.3)/2.7 = 4.70

From the z-table, it gives 0.999.. Basically 100%.

So there is a 100% chance of meeting the height requirement for women.

B) For men;

the minimum z = (57 - 67.3)/2.8 = -3.68 which gives 0.00012 from the z-distribution table.

The maximum;

z = (76 - 67.3)/2.8 = 3.07 which gives 0.9989 from the z-distribution table. Basically, 99.89%.

So there is a 99.89% chance of meeting the height requirement for men.

C) To exclude the tallest 5% of men, we need to find the z-value for 0.95 and then solve for x.

Thus from the z-table, z = 1.65. So;

1.645 = (x - 67.3)/2.8

4.606 = x - 67.3

x = 67.3 + 4.606

x ≈ 71.9 inches for men.

So, Current minimum is okay.

To exclude the shortest 5% of women, we need to find the z for 0.05.

From the z-distribution table, it has a value of -1.645. So;

-1.645 = (x - 63.3)/2.7

-4.4415 = x - 63.3

x = 63.3 - 4.4415

x ≈ 58.9 inches minimum.

So, Current maximum is okay.

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