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(4 + 5^1/2) (4 – 5^1/2) is equal to:?

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(4 + 5^1/2) (4 – 5^1/2) is equal to:?  [#permalink]

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New post 14 Dec 2019, 05:39
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A
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C
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Question Stats:

94% (00:23) correct 6% (00:28) wrong based on 32 sessions

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\((4 + \sqrt{5}) (4 – \sqrt{5})\) is equal to:

(A) –1
(B) 0
(C) 11
(D) 21
(E) 11 + 8 \(\sqrt{5}\)

Source: Master GMAT

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Re: (4 + 5^1/2) (4 – 5^1/2) is equal to:?  [#permalink]

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New post 14 Dec 2019, 05:43
SajjadAhmad wrote:
\((4 + \sqrt{5}) (4 – \sqrt{5})\) is equal to:

(A) –1
(B) 0
(C) 11
(D) 21
(E) 11 + 8 \(\sqrt{5}\)

Source: Master GMAT



Whenever you are looking at something similar to (a-b)(a+b), the answer is \(a^2-b^2\)
So \(4^2-\sqrt{5}^2=16-5=11\)

C
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Re: (4 + 5^1/2) (4 – 5^1/2) is equal to:?  [#permalink]

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New post 14 Dec 2019, 10:48
\((4 + \sqrt{5}) (4 – \sqrt{5})\) is equal to:

(A) –1
(B) 0
(C) 11 --> correct: \((4 + \sqrt{5}) (4 – \sqrt{5})\) = \((4^2 - \sqrt{5}^2)\) = 16-5=11
(D) 21
(E) 11 + 8 \(\sqrt{5}\)
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(4 + 5^1/2) (4 – 5^1/2) is equal to:?  [#permalink]

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New post 14 Dec 2019, 11:06
Its in \(a^2 - b^2\) form.

Putting the numbers in: \(4^2 -(\sqrt{5})^2\) = 16-5 = 11

Ans: C
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Re: (4 + 5^1/2) (4 – 5^1/2) is equal to:?  [#permalink]

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New post 15 Dec 2019, 05:06
SajjadAhmad wrote:
\((4 + \sqrt{5}) (4 – \sqrt{5})\) is equal to:

(A) –1
(B) 0
(C) 11
(D) 21
(E) 11 + 8 \(\sqrt{5}\)

Source: Master GMAT



-> 16 - 4√5 + 4√5 -5
->16 – 5
-> 11

so its option C
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Re: (4 + 5^1/2) (4 – 5^1/2) is equal to:?   [#permalink] 15 Dec 2019, 05:06
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