First, the concept: if X? 0? 5=a, then X is called the square root of A. The positive square root of a positive number is also called the arithmetic square root of A. ① A positive number has two square roots, one positive and one negative is opposite to each other; The square root of 20 is 0.3. Negative numbers have no square root. 2. Important formula: ①(√a)? 0? 5=a，(a≥0)； ②√a？ 0? 5=|a| III。 Properties: √(ab)=√a×√b, (a≥0, B ≥ 0); √(a/b)=√a/√b(a≥0, b>0) Fourth, the simplest quadratic condition: ① The number of roots is an integer; (2) The number of roots does not include the factor of the final root that can be opened. 5. Homogeneous secondary roots: after several secondary roots are transformed into the simplest secondary roots, if the number of roots is the same, they are homogeneous secondary roots. 6. Merge similar secondary roots: similar to merging similar items. Only add and subtract coefficients. Seven, the denominator is physical and chemical: change the root sign in the denominator, called the denominator is physical and chemical.