2. Sum and difference squared:

(a+b)2=a2+b2+2ab？

(a-b)2=a2+b2-2ab？

3. The cube of sum and difference:

a3+b3=(a+b)(a2-ab+b2)

a3-b3=(a-b)(a2+ab+b2)？

4, triangle inequality:

|a+b|≤|a|+|b|？

|a-b|≤|a|+|b|

| a |≤b & lt； = & gt-b≤a≤b？

5. Discrimination of roots

(1)b2-4ac=0 Note: The equation has two equal real roots.

(2)b2-4ac >0 Note: The equation has two unequal real roots.

(3)B2-4ac & lt； Note: The equation has no real root, but a complex number of the yoke.