AP EAMCET · Maths · Statistics

The standard deviations of $x_{i} (i = 1, 2, \dots . 10)$ and $y_{i} (i = 1, \dots 10)$ are respectively $a$ and $b$ . $\bar{x}, \bar{y}$ are the means of these two sets of observations respectively. If $z_{i} = (x_{i} - \bar{x}) (y_{i} - \bar{y})$ and $\sum_{i = 1}^{10} z_{i} = c$ , then the standard deviation of the observations $(x_{i} - y_{i}), (i = 1, 2, \dots 10)$ is

A $\sqrt{a^{2} + b^{2} + \frac{c}{5}}$
B $\sqrt{a^{2} + b^{2} - \frac{c}{5}}$
C $\sqrt{a^{2} + b^{2} - \frac{c^{2}}{5}}$
D $\sqrt{a^{2} + b^{2} + \frac{c^{2}}{5}}$

Verified Solution

Answer & Solution

Correct Answer

(B) $\sqrt{a^{2} + b^{2} - \frac{c}{5}}$

Step-by-step Solution

Detailed explanation

It is given that, zi=xi-x¯yi-y¯ And ∑i=110zi=c Rewrite the above equation. ∑i=110xi-x¯yi-y¯=c ⇒ ∑i=110xiyi-xiy¯-x¯yi+x¯·y¯=c…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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The standard deviations of xii=1,2,….10 and yii=1,…10 are respectively a and b. x¯, y¯ are the means of these two sets of observations respectively. If zi=xi-x¯yi-y¯ and ∑i=110zi=c, then the standard deviation of the observations xi-yi, i=1,2,…10 is

Step-by-step Solution