a) Ta có: \(\dfrac{5}{2}x^4\ge0\) với mọi x

\(3x^2\ge0\) với mọi x

Do đó \(\dfrac{5}{2}x^4+3x^2+9\ge9\) với mọi x

\(\Rightarrow P\left(x\right)>0\) với mọi x (đpcm)

1):

Ta có: 51 chia hết cho 3

120 chia hết cho 3

453 chia hết cho 3

=>51a+120b+453c chia hết cho 3

2):

Ta có:

A=5+5²+5³+...+5³⁰

=>A=(5+5²)+5²(5.5²)+...+5²⁸(5+5²)

=>A=(5+5²).(5²+5⁴+...+5²⁸)

Vì 5+5²=30 chia hết cho 6

=>A chia hết cho 6

1. David has a music class on Saturday. => There is a music class for David on Saturday.

ôn bài

1. Trung usually (go) goes to the library three times a week.

2. Yesterday I (be) was busy, so I (not have) didn't have time to phone you.

3. I (not eat) haven't eaten anything for two days.

4. We (not see) haven't seen her since we (be) were on holiday in Ha Long bay.

1. Her uncle/hate/go to bed early/but/she/like.

=> Her uncle hates going to bed early but she likes this.

2. We/buy/a new house/Ho Chi Minh city/a month ago.

=> We bought a new house in Ho Chi Minh city a month ago.

3. My house/small/but/my brother`s/big

=> My house is small but my brother's one is big.

(a + b)² = a² + 2ab + b²

(a - b)² = a² - 2ab + b²

a² - b² = (a + b)(a - b)

(a + b)³ = a³ + 3a²b + 3ab² + b³

(a - b)³ = a³ - 3a²b + 3ab² - b³

a³ - b³ = (a - b)(a² + ab + b²)

a³ + b³ = (a + b)(a² - ab + b²)