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# If x + y = 7 and x – y = 3, then x^2 - y^2 =

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If x + y = 7 and x – y = 3, then x^2 - y^2 = [#permalink]
Bunuel wrote:
If x + y = 7 and x – y = 3, then x^2 - y^2 =

A. -4
B. 4
C. 10
D. 21
E. 40

$$x^2 - y^2 = (x-y)*(x+y)$$

Putting the values in the equation:
$$x^2 - y^2 = 7*3 = 21$$
Option D
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Re: If x + y = 7 and x – y = 3, then x^2 - y^2 = [#permalink]
Bunuel wrote:
If x + y = 7 and x – y = 3, then x^2 - y^2 =

A. -4
B. 4
C. 10
D. 21
E. 40

x - y = 3
-> x = 3 + y

put this in the other equation ( x + y = 7):

3 + y + y = 7
-> 2y = 4 -> y = 2

Put y in the first equation (x - y = 3):

x - 2 = 3
-> x = 5

Therefore: x^2 - y^2 = 5^2 - 2^2 = 25 - 4 = 21

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Re: If x + y = 7 and x – y = 3, then x^2 - y^2 = [#permalink]
Bunuel wrote:
If x + y = 7 and x – y = 3, then x^2 - y^2 =

A. -4
B. 4
C. 10
D. 21
E. 40

pick numbers

5 +2 =7

5-2= 3

25-4= 21
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Re: If x + y = 7 and x – y = 3, then x^2 - y^2 = [#permalink]
x+y =7 and x-y=3
therefore : x+y+x-y=7+3=10
2x=10 => x=5 as x+y=7 => 5+y=7 => y=2

x^2-y^2 = 5^2-2^2 = 25-4 = 21

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Re: If x + y = 7 and x y = 3, then x^2 - y^2 = [#permalink]
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Re: If x + y = 7 and x y = 3, then x^2 - y^2 = [#permalink]
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