-Qua điểm O có thể kẻ đc một và chỉ một đường thẳng d vuông góc với đường thẳng a

-Qua điểm O có thể kẻ đc một và chỉ một đường thẳng d song song với đường thẳng a

1) Let's ..meet.. at the school gate.

2) My father ..is going to go.. fishing with his friends tomorrow.

3) It's 7.00 now I..am studying..  English in the classroom.

4) I..was.. very happy on my last vacation.

5) I hope you ..visited.. Hanoi after the harvest.

6) How ..do you feel.. now? I feel OK.

7) When ..did you buy.. this hat? A  week ago.

8) You must ..do.. your homework before watching TV.

9) Listen! The birds..is singing.. in the garden.

10) You may ..be.. interested in the traditions of different cultures.

1. My uncle __starts_a new job. It's hard work but he likes it

2. he __finishes____his homework and he can relax now

3. he __loses__his mobile phone and he doesn't know where it is

4. She __wins__a lot of money and is going to spend it on a holiday.

5. They __buy___a lot of clothes and don't have any money left

6. They __spend__all their money on computer games

7. She ___sends___an e-mail to her friend and id waiting for her frined to write back

8 He ___passes____-all his exams and his parents are very pleased with him

1/42 + 1/30 +1/20 + 1/12 + 1/6

= 1/6+ 1/12 + 1/20 + 1/30 + 1/42

= 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7

= 1/2 - 1/3 + 1/3 -1/4 + 1 /4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7

= 1/2 - 1/7

= 5/14

p là số nguyên tố mà p > 13 nên p = 3k + 1 hoặc 3k + 2 (k \(\in\) N)

- Với p = 3k + 1 ta có \(\frac{\left(3k+1\right)^2-1}{24}=\frac{9k^2+1-1}{24}=\frac{9k^2}{24}=\frac{3.3k^2}{3.8}\)chia hết cho 3, là hợp số.

- Với p = 3k + 2 ta có \(\frac{\left(3k+2\right)^2-1}{24}=\frac{9k^2+4-1}{24}=\frac{9k^2+3}{24}=\frac{3.\left(3k^2+1\right)}{3.8}\) chia hết cho 3, là hợp số.

                       Vậy suy ra điều phải chứng minh.