2. Trigonometric inequality | A+B |≤| A |+B ||||||| A |+B|| A |≤ B < = >-B≤ A ≤ B | A-B |≥| A |-B| | A |≤

3. The solution of the unary quadratic equation -b+√(b2-4ac)/2a -b-√(b2-4ac)/2a.

4. Relationship between root and coefficient X1+x2 =-b/ax1* x2 = c/a Note: Vieta theorem discriminant b2-4ac=0 Note: the equation has two equal real roots b2-4ac >.