Is 2 ≠ 1, or is it?

Algebra Level 2

Assuming that a = b, we can construct an equation such that it seems as if it proves 2 = 1.

a = b \boxed {a = b}

  1. a 2 = b × a a^{2} = b \times a

  2. a 2 b 2 = b a b 2 a^{2} - b^{2} = ba - b^{2}

  3. ( a b ) ( a + b ) = b ( a b ) (a - b)(a + b) = b(a - b)

  4. ( a b ) ( a + b ) ( a b ) = b ( a b ) ( a b ) \frac {(a - b)(a + b)} {(a - b)} = \frac {b(a - b)} {(a - b)}

  5. a + b = b a + b = b

  6. 2 b = b 2b = b

Where is the flaw in logic (mistake), if there is one?

The mistake is in the third line There is no mistake The mistake is in the fourth line The final line is wrong, so it proves that everything else is too

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1 solution

The reason the flaw is in the fourth line is because a = b a = b , and as a b = 0 a - b = 0 , the equation changes to: ( a b ) ( a + b ) 0 = b ( a b ) 0 \frac {(a - b)(a+b)} {0} = \frac {b(a - b)} {0} The mistake then arises there, because as we all know...

Dividing with zero just doesn't work!

You should mention the number for each line. P.S. I love the fourth option haha!

Mahdi Raza - 1 year, 1 month ago

Yeah. Maybe I should. Thanks for the idea!

A Former Brilliant Member - 1 year, 1 month ago

Good Work Keep it up.. :)

Anushath Mohamed - 1 year, 1 month ago

Wait. Aren't there mistakes in the fourth, fifth and the last line?

B D - 1 year ago

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What do you mean? Where's the mistake?

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a+b doesn't equal b

B D - 1 year ago

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@B D That's the reality of the 4th line

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@A Former Brilliant Member Isn't a+b=b on the fifth line?

B D - 1 year ago

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@B D The result of the 4th line. Check it again

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@A Former Brilliant Member I think I got the logic.

B D - 1 year ago

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@B D Glad I helped!

@A Former Brilliant Member Because you can't divide by 0 everything and it is like more important.

B D - 1 year ago

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@B D Yes. That is why the other lines are wrong too.

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