Inspired By Omkar Kulkarni

Algebra Level 2

9 × 4 = ? \large \sqrt {-9} \times \sqrt {-4} = \, ?

36 \sqrt{36} 6i 6 -6

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2 solutions

Mehul Arora
May 23, 2015

a × b = a b \sqrt{a} \times \sqrt {b}= \sqrt {ab} is only valid for ( a , b ) (a,b) are non negative integers.

Hence, We have to bring complex numbers into play.

i 9 × i 4 \rightarrow i \sqrt{9} \times i \sqrt{4}

Here, i is a complex number which satisfies i 2 = 1 {i}^{2}=-1

i 2 × 3 × 2 \rightarrow {i}^{2} \times {3} \times {2}

6 \rightarrow \boxed {-6}

Which is our answer.

Cheers!

Good solution

Sai Ram - 5 years, 10 months ago

I don't understand what you mean by the following statement:

a × b \sqrt{a} \times \sqrt {b} is only valid for ( a , b ) (a,b) are positive integers.

Did you mean to say that a × b = a b \sqrt a\times \sqrt b\color{#D61F06}{=\sqrt{ab}} is valid only for non-negative reals a , b a,b ?

EDIT: This was a bit flawed since the "valid only" term is not necessarily true (can be seen in the counter-example I gave).

Prasun Biswas - 6 years ago

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Arghh. Sorry :3

Mehul Arora - 6 years ago

Edited. Thanks.

Mehul Arora - 6 years ago

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Check again. It's not fixed yet. That identity holds for all non-negative reals a , b a,b , not just for positive integers a , b a,b .

Prasun Biswas - 6 years ago

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@Prasun Biswas Fine now?? :3 :3 :3

Mehul Arora - 6 years ago

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@Mehul Arora Wait, I think I have a problem with the "valid only" part. That identity does hold sometimes when one of a a or b b is negative. Let me give the counter-example in another comment.

Prasun Biswas - 6 years ago

Here's a counter-example to your "only valid for" claim. Consider a = ( 4 ) a=(-4) and b = 9 b=9 . Now, we have,

a b = 36 = 6 i and a × b = 2 i × 3 = 6 i \sqrt{ab}=\sqrt{-36}=6i\quad\textrm{and}\quad\sqrt{a}\times\sqrt{b}=2i\times 3=6i

You can see that the identity holds even though a a is negative. I would say that you should edit your solution to reflect that the identity you stated holds for all non-negative reals but fails to hold for some non-negative reals.

Prasun Biswas - 6 years ago
Nihar Mahajan
May 23, 2015

9 × 4 = 1 9 × 1 4 = 1 ( 2 2 3 2 ) = 6 \huge \sqrt{-9} \times \sqrt{-4} \\\huge = \sqrt{-1} \sqrt{9} \times \sqrt{-1} \sqrt{4} \\ \huge= -1(\sqrt{2^23^2}) \\ \huge= \boxed{-6}

You forgot Cheers xD

Mehul Arora - 6 years ago

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Oops!

C h e e r s ! \Huge Cheers!

Nihar Mahajan - 6 years ago

Good solution , upvoted

Sai Ram - 5 years, 10 months ago

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