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Why do we use b^2 -4ac=0 to get the answer?
There are 3 types of roots for quadratic equation
i) Real and distinct roots
ii) Equal roots
iii) Unequal or unreal roots
For i), your quadratic equation has 2 distinct roots (or answers). Your b^2-4ac is more than 0
For ii), your quadratic equation has equal roots, meaning when you solve them, the answers are the same. So your b^2-4ac is equal to 0
For iii), your quadratic equation has no answers. I think it is not in your syllabus so I would avoid talking about that.
For more information you can read https://perfectmaths.files.wordpress.com/2011/07/discriminant-table.png
i) Real and distinct roots
ii) Equal roots
iii) Unequal or unreal roots
For i), your quadratic equation has 2 distinct roots (or answers). Your b^2-4ac is more than 0
For ii), your quadratic equation has equal roots, meaning when you solve them, the answers are the same. So your b^2-4ac is equal to 0
For iii), your quadratic equation has no answers. I think it is not in your syllabus so I would avoid talking about that.
For more information you can read https://perfectmaths.files.wordpress.com/2011/07/discriminant-table.png
The b2 - 4ac thing is meant to be learnt in sec 3 only.