đoạn thay sina=2cosa xong như nào ạ

Cho Tam giác ABC vuông tại A, đường cao AH vuôgn gó với BC . Biết AB= 6cm,CH= 6,4cm. a) Tính BH. b) Tính AC. 
cíu tui vsss
 

cho e hỏi dòng 2: 6² / HB + 6,4 lm thế nào ra đc HB² + 6,4HB - 36 = 0

Giúp mình vs, mìnk cần gấp ạ

1. I think we should abolish all these antiquated customs (done)

2. The house shouldn't be left unlocked for any reason (account)

3. What should one do in such cases? (be)

4. Do you regret not learning to swim? (wish)

5. ''Please, please let me go to the party tonight!'' he asked them (begged)

Mọi ng ơi, giúp mìnk vs!!!

1.I usually (go) …………………. fishing on Saturdays.

2.My brother used to ( go )……………… to work by motorbike. Now he cycles.

3.How about (see) ……………. a movie on Sunday evening ?

4.Nam didn’t use (play) ……………………. football in the afternoon. Now he often does it.

5.Nam (learn) …………………………….. English for 8 years.

6.Last night, my father ( watch) ………………………..TV.

7.Ba (visit)………………….. to Ha Long Bay this summer vacation.

8. I like ( work) ……………………….. as a volunteer for that orphanage.

9. Mai’s mother is clever . She usually (make)………………..………..lovely things.

10.You (buy) ………………………………..a new bike yesterday?.

11. Last night we (go) ……………………….……to Lan’s birthday party.

12. We (work)……………………………….…..in the garden yesterday morning.

13.Would you mind ( open ) …………….. the windows ?

14.Do you mind if I ( take) ……… a photo with you ?

15.Would you mind if I (open)…………… the door ?

mong các gia sư giải thích chi tiết giúp e vs e cần gấp ạk!!!