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Algebra Level 1

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............................................................................................. \text{.............................................................................................}

What is 11 8 ? 11-8\text{?}

i = 1 7 i ( 1 ) i + 1 \sum_{i=1}^7i(-1)^{i+1} 6 9 6-\sqrt{9} 20 1 4 ( m o d 5 ) 201^4 \pmod{5} Two

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4 solutions

William Cui
Jan 17, 2014

If we look at a number line, we can start at 11 and count back 8 spaces to arrive at 3. However, which choice is equivalent to 3?

Obviously, the choice Two \text{Two} can be eliminated, as 2 3 2\neq 3

20 1 4 201^4 has a units digit of 1, and therefore is equal to 1 ( m o d 5 ) 1\pmod 5 , and since 1 3 1\neq 3 , this choice can also be eliminated.

For the summation choice,

i = 1 7 i ( 1 ) i + 1 \sum_{i=1}^{7}i(-1)^{i+1}

( 1 ) 2 + 2 ( 1 ) 3 + 3 ( 1 ) 4 \implies (-1)^2+2\cdot (-1)^3+3\cdot (-1)^4\ldots

1 2 + 3 4 + 5 6 + 7 = 4 \implies 1 - 2 + 3 - 4 + 5 - 6 + 7 = 4

And since 3 4 3\neq 4 , this choice is clearly not correct.

So, obviously, the last choice, 6 9 6-\sqrt{9} , must be correct. Testing it, we find that 6 9 = 6 3 = 3 6-\sqrt{9}=6-3=\boxed{3}\ \blacksquare , so that is our desired answer.

Seriously 32% people got this wrong?

Aryan Gaikwad - 6 years, 3 months ago

This is a fail question -> The answer can be : 6 - (-3) = 9 or 6- (+3) = 3 --------------

Victor Marinho - 6 years, 2 months ago

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Nope... Sqrt (9)=3 and only 3. A sqrt is always positive, we say that sqrt(x^2)=|x|. Try ploting the function y=sqrt(x) and you shall see.

Rodrigo Livianu - 6 years, 2 months ago

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My maths teacher taught me that the sqrt(9) = +3 and -3.

Oliver Daniel - 6 years ago

6-\sqrt(9)

I don't get this Latex thing

6 9 6-\sqrt { 9 }

oh, that's how it's done

Michael Mendrin - 7 years, 2 months ago

I still remember the math from elementary that 11 - 8 is 3

M Nuurussubchiy Fikriy - 7 years, 1 month ago

3 is correct Answer.

Rubina Sagani - 6 years, 5 months ago
MadiHa Aziz
Jan 21, 2014

6-3=3

there is no difficulty at all...we are ver y sure that the answer is 3 we just have to be confident here and the option 6-sqrt(9) will fetch us the right answer....

Avi Agrawal
Dec 30, 2014

9^1/2 =23 So, 6 - 9^1/2 =3. And, 11 - 8 is also = 3.. So the ans. is '6 - 9^1/2'

how can 9^1/2 = 23? it should be 3. is this a typo?

Razik Ridzuan - 6 years, 2 months ago

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