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The mean and sd for a,b and 2 and 2/√3 respectively, thr value of ab would be?
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The mean and sd for a,b and 2 and 2/√3 respectively, thr value of ab w...
So you have three values, which are : a, b, and 2. 

The mean of the three values is 3. 

So to determine that expression, add up the values and divide by 3 (the number of values): 

(a + b + 2) / 3 = 3 

simplify: 

a + b + 2 = 9 
a + b = 7 

Now let's go to the standard deviation. 

To solve this, you take the difference of each data point away from the mean, square them, average those values, then get the square root. we are told the result is 2√3, so we have: 

(a - 3)x + (b - 3)x + (2 - 3)x
(a - 3)x + (b - 3)x+ (-1)x 
(a - 3)x + (b - 3)x + 1 

Divide that by 3: 

[(a - 3)x + (b - 3)x + 1] / 3 

And get the square root: 

√{[(a - 3)x + (b - 3)x + 1] / 3} 

Let's rationalize that by multiplying both halves of the fraction by √3: 

√{3[(a - 3)x + (b - 3)x + 1] / 9} 
√{3[(a - 3)x + (b - 3)x + 1]} / 3 

This value is equal to 2√3, so let's set them equal: 

√{3[(a - 3)x + (b - 3)x + 1]} / 3 = 2√3 

simplify, starting with multiplying both sides by 3: 

√{3[(a - 3)x + (b - 3)x + 1]} = 6√3 

Now squaring both sides: 

3[(a - 3)x + (b - 3)x + 1] = 36 * 3 

From here, let's divide both sides by 3: 

(a - 3)x + (b - 3)x + 1 = 36 

Subtract 1 from both sides: 

(a - 3)x + (b - 3)x = 35 

Now we can use the first equation and substitute an expression for a in terms of b, then solve for b: 

a + b = 7 
a = 7 - b 

So we have: 

(7 - b - 3)x + (b - 3)x = 35 
(4 - b)x + (b - 3)x = 35 

Square the binomials: 

16 - 8b + bx + bx - 6b + 9 = 35 

and simplify: 

2bx - 14b + 25 = 35 
2bx - 14b - 10 = 0 

Divide both sides by 2: 

bx - 7b - 5 = 0 

Quadratic Formula: 

b = [ -b x √(bx - 4ac)] / (2a) 
b = [ -(-7) x √((-7)x - 4(1)(-5))] / (2 * 1) 
b = [ 7 x √(49 + 20)] / 2 
b = [ 7 x √(69)] / 2 

So we have two values for b, so let's see what we get for a: 

a = 7 - b 
a = 7 - [ 7 - √(69)] / 2 and a = 7 - [ 7 + √(69)] / 2 
a = 7 - [ 7/2 - √(69) / 2] and a = 7 - [ 7/2 + √(69) / 2] 
a = 7 - 7/2 + √(69) / 2 and a = 7 - 7/2 - √(69) / 2 
a = 14/2 - 7/2 + √(69) / 2 and a = 14/2 - 7/2 - √(69) / 2 
a = 7/2 + √(69) / 2 and a = 7/2 - √(69) / 2 

So a and b end up being the same values. 

Before going on, as a test, let's solve for the mean and SD, using √69 approx 8.307, we get: 

a = 7/2 + 8.307 / 2 and b = 7/2 - 8.307 / 2 
a = 3.5 + 4.1535 and b = 3.5 - 4.1535 
a = 7.6535 and b = -0.6535 

Mean: 

(7.6535 - 0.6535 + 2) / 3 
9/3 

Mean works out. Now SD: 

(7.6535 - 3)x + (-0.6535 - 3)x + (2 - 3)x 
4.6535x + (-3.6535)x + (-1)x 
21.65506225 + 13.34806225 + 1 
36.0031245 

Divide that by 3: 

36.0031245 / 3 = 12.0010415 

Square root of that: 

√12.0010415 = 3.46425 

And 2√3 = 3.464101 

Not exact, but since we rounded mid-way through, wouldn't be, but is close enough to be considered correct. 

So now that we have values for a and b, we can solve for: 

ab 
[ 7 - √(69)] / 2 * [ 7 + √(69)] / 2 
(7 - √69)(7 + √69) / 4 
(49 + 7√69 - 7√69 - 69) / 4 
(49 - 69) / 4 
(-20) / 4 

-5
This question is part of UPSC exam. View all Quant courses
Most Upvoted Answer
The mean and sd for a,b and 2 and 2/√3 respectively, thr value of ab w...
The mean and standard deviation (SD) of two variables, a and b, are given as 2 and 2/√3, respectively. We need to determine the value of the product ab. Let's break down the problem step by step.

Step 1: Understanding Mean and Standard Deviation
Mean is a measure of central tendency that represents the average value of a set of numbers. Standard deviation measures the dispersion or spread of the data points around the mean. It provides information about how much the individual data points deviate from the mean.

Step 2: Calculating Variance
To find the product of a and b, we first need to calculate the variance of each variable. Variance is the square of the standard deviation.

The variance of a is equal to the square of its standard deviation:
Variance of a = (Standard deviation of a)^2
= (2)^2
= 4

The variance of b is equal to the square of its standard deviation:
Variance of b = (Standard deviation of b)^2
= (2/√3)^2
= (4/3)

Step 3: Understanding Covariance
To find the product ab, we need to understand the concept of covariance. Covariance is a measure of how much two variables change together. It gives us information about the relationship between two variables.

Step 4: Calculating Covariance
The covariance of a and b is given by the formula:
Covariance(a, b) = (Variance of ab) - (Mean of a * Mean of b)

Step 5: Finding the Value of Covariance
Since the mean of a and b are given as 2, the covariance of a and b can be calculated as:
Covariance(a, b) = (Variance of ab) - (2 * 2)

We know that the variance of a is 4, and the variance of b is 4/3. Therefore, we can rewrite the equation as:
Covariance(a, b) = (Variance of ab) - 4

Step 6: Solving for Variance of ab
Rearranging the equation, we get:
(Variance of ab) = Covariance(a, b) + 4

Step 7: Determining the Value of ab
To find the value of ab, we need to calculate the variance of ab first. Then, we can substitute the value of variance of ab into the equation:
ab = (Variance of ab) + 4

Step 8: Substitute Values and Calculate
Using the given mean and standard deviation, we substitute the values into the equation and solve for ab:
ab = (Covariance(a, b) + 4) + 4
ab = Covariance(a, b) + 8

Step 9: Final Answer
Since the covariance value is not provided in the given information, we cannot calculate the exact value of ab. However, we have derived the equation ab = Covariance(a, b) + 8, which gives us a general formula to calculate the product of a and b once the covariance value is known.

Therefore, the value of ab depends on the covariance between variables
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The mean and sd for a,b and 2 and 2/√3 respectively, thr value of ab would be?
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