If 7x + 3y = 12 and 3x + 7y = 8, then x y = ?

Question

If  7x + 3y = 12  and  3x + 7y = 8 , then  x – y =

(A) 1
(B) 3
(C) 7
(D) 8
(E) 12

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Purnank · Answer

( 7x + 3y = 12 ) - ( 3x + 7y = 8 ) = ( 4x - 4y = 4 )

taking 4 common and cancelling on each side.

x – y = 1

Answer is A.

Ayeka · Answer

7x+3y=12 —————(1)
3x+7y=8 —————-(2) 
Find: x-y

Eq(1)-Eq(2) 
7x-3x+3y-7y=12-8 
4x-4y=4 
x-y=1

ANSWER: A

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Bismuth83 · Answer

1. The LHS (left hand side) of both equations are the same but flipped. We need to  find the value of x - y , which is symmetrical.

2. To do this, we can either add or subtract the 2 equations. Since x and y have different signs, it's more reasonable to subtract.

3. So, we have:  4 = (12) - (8) = (7x + 3y) - (3x + 7y) = 4x - 4y = 4(x - y) \rightarrow     x - y = 1

prantorboni · Answer

7 x + 3 y = 12 
(-)3 x + (-)7 y = (-)8 
---------------------
 4 x  − 4 y = 4
 4(x-y) = 4
 (x-y) = 1

Answer: A.

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