Giải chi tiết giúp mình tại sao `2cosx-sinx=0 ⇔ tanx=2` với ạ

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1- We haven’t seen the car before.  It’s the………………………………………………………………………… 2- I haven’t gone there before.  It’s the ………………………………………………………………………… 3- They haven’t performed here before.  It’s the………………………………………………………………………… 4- Nam hasn’t held a girl’s hand before.  It’s the………………………………………………………………………… 5- Duy hasn’t read this kind of story before.  It’s the………………………………………………………………………… 6- She started to dance ten years ago.  She has danced for ten years. ( ten years la khoảng thời gian vì thế ta dùng for) 7- They began to smoke 20 years ago.  They have………………………………………………………………………… 8- Mr Doone started to work here when he graduated.  Mr Doone has………………………………………………………………………… 9- The nice girl began to dance in 1990.  The nice girl has……………………………………………………………………… 10- We began to study English when we were too young.  We have been………………………………………………………………………… 11- How long have you learnt English?  How long ago did you begin to learn English? 12- How long have you painted the door?  When …………………………………………………………………………? 13- How long have you been jogging?  When …………………………………………………………………………? 14- When did you begin to speak Cambodia?  How long …………………………………………………………………………? 15- When did you begin to listen to Big Bang?  How long …………………………………………………………………………? 16Samuel started keeping a diary in 1997.  Samuel has ___________________________________________________________ 17. We started leaming English fìve years ago.  We have _____________________________________________________________ 18. I began living in Ho Chi Minh City when I was 8.  I have _______________________________________________________________ 19. She has taught the children in this remote village for five months.  She started ___________________________________________________________ 20. How long is it since you rode a bicyde?  When _______________________________________________________________