how 2*2 can be equal to 5?

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- Feb 2nd 2010, 11:20 PM #1

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- Feb 3rd 2010, 12:37 AM #2

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- Feb 3rd 2010, 12:47 AM #3
Hello afsanehIt's when you take the square root that you need to be careful. Just because $\displaystyle a^2 = b^2$, it doesn't necessarily mean that $\displaystyle a = b$. It could be that $\displaystyle a = -b$.

It is true that $\displaystyle (2^2-\tfrac92)^2 = (5 - \tfrac92)^2$, the line I've coloured red. This is because, when we evaluate the expressions in each bracket, the one on the LHS is $\displaystyle -\tfrac12$ and the one on the RHS is $\displaystyle \tfrac12$. Squaring each of these gives the same answer: $\displaystyle \tfrac14$. But that doesn't mean that they are equal, does it?

So remember: when you take a square root, remember the possible minus sign!

Grandad

- Feb 3rd 2010, 01:42 AM #4

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