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# For real numbers x and y, define x?y = (x + y)(x - y). What is 3?(4?5)

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Joined: 02 Sep 2009
Posts: 58353
For real numbers x and y, define x?y = (x + y)(x - y). What is 3?(4?5)  [#permalink]

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26 Mar 2019, 05:21
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For numbers x and y, define x?y = (x + y)(x - y). What is 3?(4?5) ?

(A) -72
(B) -27
(C) -24
(D) 24
(E) 72

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Posts: 16
Re: For real numbers x and y, define x?y = (x + y)(x - y). What is 3?(4?5)  [#permalink]

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26 Mar 2019, 06:54
Following the steps through we get first 3?(-9) and as a result -72.

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Re: For real numbers x and y, define x?y = (x + y)(x - y). What is 3?(4?5)  [#permalink]

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26 Mar 2019, 08:01
4?5 is -9 and 3?-9 (9-81, x^2-y^2) gives -72. Option A

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Re: For real numbers x and y, define x?y = (x + y)(x - y). What is 3?(4?5)  [#permalink]

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26 Mar 2019, 11:15
Bunuel wrote:
For numbers x and y, define x?y = (x + y)(x - y). What is 3?(4?5) ?

(A) -72
(B) -27
(C) -24
(D) 24
(E) 72

solve for the expression as per expression we get
3?(-9)
then
-72
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Re: For real numbers x and y, define x?y = (x + y)(x - y). What is 3?(4?5)  [#permalink]

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27 Mar 2019, 19:11
Bunuel wrote:
For numbers x and y, define x?y = (x + y)(x - y). What is 3?(4?5) ?

(A) -72
(B) -27
(C) -24
(D) 24
(E) 72

We see that x?y is x^2 - y^2,

Thus, 4?5 = 4^2 - 5^2 = 16 - 25 = -9, and therefore,

3?(-9) = 3^2 - (-9)^2 = 9 - 81 = -72

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Re: For real numbers x and y, define x?y = (x + y)(x - y). What is 3?(4?5)   [#permalink] 27 Mar 2019, 19:11
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