How to use FOIL
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GRE Quantitative Reasoning › How to use FOIL
Quantity A:
Quantity B:
Quantity B is greater.
Quantity A is greater.
The two quantities are equal.
The relationship cannot be determined.
Explanation
To approach this problem, consider the two quantities
Quantity A:
Quantity B:
They are in different forms, so expand quantity A:
Quantity A:
Quantity B:
Now, for the purpose of comparison, subtract shared terms from each quantity:
Quantity A*:
Quantity B*:
Both and
are negative, non-zero values. Since
is a product of two negative values, it must be positive. Quantity B must be greater than Quantity A.
Expand the function:
Explanation
Use the method of FOIL (First, Outside, Inside, Last) and add exponents for like bases:
Quantity A:
Quantity B:
Quantity A is greater.
Quantity B is greater.
The two quantities are equal.
The relationship cannot be determined from the information given.
Explanation
The difference of squares formula says (x + a)(x - a) = x2 - a2.
Thus, Quantity A equals 8.
Therefore, Quantity B is greater.
Quantity A:
Quantity B:
The relationship cannot be determined.
Quantity A is greater.
Quantity B is greater.
The two quantities are equal.
Explanation
Begin by expanding Quantity A:
Now in order to compare this to Quantity B:
A good method would be to subtract shared terms from each Quantity; in this case, both quantities have an and
term. Removing them gives:
Quantity A' :
Quantity B' :
The question now is the sign of Quantity A'; if it's always positive, Quantity A is greater. If it's always negative, Quantity B is greater. If it is zero, the two are the same.
We only know that
If , then Quantity A' would be zero.
If , then Quantity A' would be positive.
Since values of x and y can be chosen to vary the relationship, th relationship cannot be determined.
What is the value of t if: 3x2 + tx - 21 = (3x - 3)(x + 7)?
-3
-18
21
24
18
Explanation
Use the foil method: (3x - 3) (x + 7) = 3x2 +21x - 3x - 21 = 3x2 +18x -21 so t = 18.
Solve the following expression, .
Explanation
You must FOIL the expression which means to multiply the first terms together followed by the outer terms, then the inner terms and lastly, the last terms.
The expression written out looks like
.
You multiple both First terms to get .
Then the outer terms are multiplied .
Then you multiple the inner terms together .
Finally you multiply the last terms of each .
This gives you or
.
Expand the following equation:
Explanation
use FOIL to factor the expression.
First: (x3)(x) = x4
Outside (x3)(7) = 7x3
Inside (–3)(x) = –3x (Don't forget the negatives!)
Last (3)(7) = –21
Quantity A:
Quantity B:
The relationship cannot be determined.
Quantity A is greater.
Quantity B is greater.
The two quantities are equal.
Explanation
This problem is deceptive. Looking at Quantity A, one may think to factor and reduce it as follows:
Which is identical to Quantity B.
However, we cannot ignore that in the original fraction! We are given no conditions as to the value of
. If
, then Quantity A would be undefined. Since we're not given the condition
, we cannot ignore this possibility.
The relationship cannot be established.
Quantity A:
Quantity B:
Quantity B is greater.
Quantity A is greater.
The two quantities are equal.
The relationship cannot be determined.
Explanation
Begin by expanding Quantity A:
Now in order to compare this to Quantity B:
A good method would be to subtract shared terms from each Quantity; in this case, both quantities have an and
term. Removing them gives:
Quantity A' :
Quantity B' :
The question now is the sign of Quantity A'; if it's always positive, Quantity A is greater. If it's always negative, Quantity B is greater. If it is zero, the two are the same.
We know that
Now compare and
:
Looking at absolute values so that we're only considering positive terms:
From this it follows that by multiplying across the inequality :
From this we can determine that the magnitude of is greater. However, since this is the product of one negative number and two positive numbers,
is negative, and the sum of
and
must in turn be negative, and so Quantity A' must be negative!
From this we can say that Quantity B is greater.
Quantitative Comparison
Quantity A:
Quantity B:
Quantity A is greater.
Quantity B is greater.
The two quantities are equal.
The relationship cannot be determined from the information given.
Explanation
Quantity A: 22 + 32 = 4 + 9 = 13
Quantity B: (2 + 3)2 = 52 = 25, so Quantity B is greater.
We can also think of this in more general terms. _x_2 + _y_2 does not generally equal (x + y)2.