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C1 discriminant question watch

1. for part d of this question. I got to the part 'k^2-12<0 ' and I don't understand why you can just add 12 on both side and just square root 12. I was told that you can't square root numbers when there is an inequality sign there. Thanks.
2. (Original post by coconut64)
for part d of this question. I got to the part 'k^2-12<0 ' and I don't understand why you can just add 12 on both side and just square root 12. I was told that you can't square root numbers when there is an inequality sign there. Thanks.
you can and you get 2 answers
3. Sketch a graph of y=k2-12 and you will see that for y<0 , k has to be between 2 values. .
4. (Original post by B_9710)
Sketch a graph of y=k2-12 and you will see that for y<0 , k has to be between 2 values. .
Thats true. Thats in one of the past paper. I vaguely remembered my teacher telling me that you can't square root it when it has an inequality sign though. So when can you square it root and when can you not? thanks.
5. (Original post by coconut64)
Thats true. Thats in one of the past paper. I vaguely remembered my teacher telling me that you can't square root it when it has an inequality sign though. So when can you square it root and when can you not? thanks.
Well have a little think about it. If we have you cannot just square root it to get . You have to think about the graph of and realise that this means that and from the graph you can clearly see that this only occurs when .
Generally when inequalities are involved you have to take extra caution than in an equation.
You would have to take special care when solving
(random example). In this example you could not just multiply by (x-5) as if (x-5) was negative then it would flip the inequality sign. You do not need to worry about the second example I gave this will not be tested in standard A level maths.
Main point is take extra care when dealing with inequalities.

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