Ta có

\(\frac{B M}{A M} = \frac{B C}{A C} = \frac{a}{b}\) (Theo định lý đường phân giác)

\(\frac{C N}{A N} = \frac{B C}{A B} = \frac{a}{b}\) (Theo định lý đường phân giác)

\(\Rightarrow \frac{B M}{A M} = \frac{C N}{A N} \Rightarrow \frac{B M}{C N} = \frac{A M}{A N}\) => MN//BC (theo định lý Ta-let)

\(\Rightarrow \frac{A M}{A B} = \frac{M N}{B C} \Rightarrow \frac{A M}{b} = \frac{M N}{a}\)  (1)

Ta có

\(\frac{A M}{B M} = \frac{A C}{B C} = \frac{b}{a}\) (TTheo định lý đường phân giác)

\(\Rightarrow \frac{A M}{b} = \frac{B M}{a} = \frac{A M + B M}{a + b} = \frac{A B}{a + b} = \frac{b}{a + b}\)

\(\Rightarrow A M = \frac{b^{2}}{a + b}\) Thay vào (1)

\(\Rightarrow \frac{\frac{b^{2}}{a + b}}{b} = \frac{M N}{a} \Rightarrow \frac{b}{a + b} = \frac{M N}{a} \Rightarrow M N = \frac{a b}{a + b}\)

Vì tam giác ABC cân tại A nên :

AB = AC = 12

Xét tam giác ABC, ta có:

CD là đường phân giác của góc C mà D thuộc AB nên

AD/DB = AC/BC

AD/DB=12/6

AD/DB = 2

AD= 2DB

Mà D nằm trên đoạn AB nên :

AD + DB = AB

2DB + DB = 12

3DB=12

DB=4 (cm)

vậy AD = 2 x 4 =8(cm)

SUY RA AD = 8 (cm) và DB =4 (cm)

Topic 3 : I usually try to help my parents with housework. Every day, I clean the house or take care of the baby. After school, I usually prepare dinner and wash the dishes afterward. On weekends, I take care of the plants in the garden. Helping my family makes me feel more responsible, happy, and joyful. I will help my parents so they can be proud and have an easier life.

1. The Muong earn a living by farming and raising animals.

2. They are in the habit of exercising after work.

1. If you don't drink more water, you will get more acne.

2. My sister wants to see the terraced fields in Sa Pa.

3. He spend time on his phone , he will complete his work.

a) ( - 12x¹³y¹⁵ + 6x¹⁰y¹⁴ ) : ( -3x¹⁰y¹⁴ )
=4x³y - 2
b) ( x - y ) ( x²- 2x + y ) - x³+ x²y
= x³ - 2x² + xy - x²y + 2xy - y²- x³ + x²y
= 3xy - 2x² - y²