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4 years ago

Under what circumstances are composite primary keys appropriate?

1 answer:

8 0

A composite primary key is a set of several primary keys that, together, uniquely identifies each record.
Primary keys are used as identifiers of composite entities. In this case each primary key combination is allowed only once in the *:* relationship.
Other case where primary keys are used is as identifiers of weak entities, where the weak entity has a strong identifying relationship with the parent entity.

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3 years ago

Two machines can finish a job in StartFraction 20 Over 9 EndFraction hours. Working alone, one machine would take one hour long

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Answer

5 hours

Explanation

The two working together can finish a job in

$\frac{20}{9} \: hours$

Also, working alone, one machine would take one hour longer than the other to complete the same job.

Let the slower machine working alone take x hours. Then the faster machine takes x-1 hours to complete the same task working alone.

Their combined rate in terms of x is

$\frac{1}{x} + \frac{1}{x - 1}$

This should be equal to 20/9 hours.

$\frac{1}{x} + \frac{1}{x - 1} = \frac{9}{20}$

Multiply through by;

$20x(x - 1) \times \frac{1}{x} +20x(x - 1) \times \frac{1}{x - 1} = 20x(x - 1) \times \frac{9}{0}$

$20(x - 1) +20x = 9x(x - 1)$

$20x - 20+20x = 9{x}^{2} - 9x$

$9{x}^{2} - 9x - 20x - 20x + 20= 0$

$9{x}^{2} - 49x + 20= 0$

Factor to get:

$(9x - 4)(x - 5) = 0$

$x = \frac{4}{9} \: or \: x = 5$

It is not feasible for the slower machine to complete the work alone in 4/9 hours if the two will finish in 20/9 hours.

Therefore the slower finish in 5 hours.

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3 years ago

Which password attack method uses long lists of words that have been predefined and can be quickly downloaded for use to break a

vova2212 [387]

Answer:

A dictonary attack.

Explanation:

Dictionaries hold many words that can be downloaded and used. Word or name passwords are usually just one word or maybe two. In other words, predefined words being used as a password? Not safe. When someone attempts to crack it, they use brute force attacks. Among these are dictionary attacks, which focus on the actual words rather than numbers.

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2 years ago