If a + b = x and a - b = y , then 3ab =

Question

A. (3x^2 - 3y^2)/4
B. (3y^2 - 3x^2)/4
C. 3x+3y/2
D. 3x - 3y/2
E. 3x^2 - 3y^2/2

can someone explain!

Bunuel · Accepted Answer

Given:  a + b = x  and  a - b = y .

Sum the equations:  2a=x+y .
Subtract:  2b=x-y .

Multiply:  4ab=x^2-y^2  --> multiply by 3/4:  3ab=\frac{3x^2-3y^2}{4} .

Answer: A.

Zarrolou · Answer

a=x-b 
 a=y+b

x-b=y+b   b=\frac{x-y}{2}

b=x-a 
 b=a-y

x-a=a-y   a=\frac{x+y}{2}

3ab=3(\frac{x^2-y^2}{4}) 
A

fozzzy · Answer

Great Explanation! Thanks....

Virgilius · Answer

Interesting question. Let's try a different approach.

You should recall the following two formulas :

\((a+b)^2 = a^2 + 2ab + b^2\)
\((a-b)^2 = a^2 - 2ab + b^2\)

Using what is given in the question stem, we get :

\(a^2 + 2ab + b^2 = (a+b)^2 = x^2\)
\(a^2 - 2ab + b^2 = (a-b)^2 = y^2\)

Substracting the two equations yields : \(4ab = x^2 - y^2\) meaning that \(ab = \frac{(x^2 - y^2)}{4}\).

Multiply the result by 3 and we get \(3ab = \frac{3(x^2 - y^2)}{4}\)

Which is answer choice A.

Hope that helped :-D

atalpanditgmat · Answer

plugging numbers is quite fast in this case:
Let 
a=1
b=2

Then; x= 1+2=> 3 and Y= 1-2=> -1

Question asks 3ab=.........
As our number 3ab= 3*1*2=6

now plug the value of x and y in the answer choices. Option A gives 6 and that is the number we need.

Hence: Ans is A.

Atal Pandit

Bounce1987 · Answer

a=x+y/2; b=x-y/2;3ab=3(x^2-y^2/4)= A

jkolachi · Answer

Excellent question. saving it in my notebook to try later.

pushpitkc · Answer

Plugging values is another method, you can use to solve this one!
Let a=3, b=2, x = 3+2 = 5
Similarly y = 3-2 = 1

We know that 3ab = 3*2*3 = 18

Of the answer options,
 A. (3x^2 - 3y^2)/4 = 3(25) - 3(1)/4 = 75 - 3/4 = 18 
B. (3y^2 - 3x^2)/4 = 3(1) - 3(25)/4 = 3 - 75/4 = -18
C. 3x+3y/2 = 3(5) + 3(1)/2 = 9
D. 3x - 3y/2 = 3(5) - 3(1)/2 = 6
E. 3x^2 - 3y^2/2 = 3(25) - 3(1)/2 = 73 - 3/2 = 36

Hence, Option A is the answer. 
If you started with evaluating the first choice, it wouldn't take much time to find the answer.

bumpbot · Answer

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If a + b = x and a - b = y , then 3ab =

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