Bài 1 : Tìm chữ số hàng đơn vị của số a, biết :

1) a = 3²⁰¹⁹.7²⁰¹⁰.13²⁰¹¹

2) a = 3²⁰²⁰.7²⁰¹⁹.13²⁰¹⁸

Bài 2 : Cho tỉ lệ thức \(\frac{a}{b}=\frac{c}{d}\). Chứng minh rằng

a) \(\frac{ac}{bd}=\frac{a^2+c^2}{b^2+d^2}\)

b) \(\frac{ad}{cd}=\frac{a^2-b^2}{c^2-d^2}\)

c) \(\frac{ad}{cd}=\frac{\left(a-b\right)^2}{\left(c-d\right)^2}\)

Ex 1: The past simple tense or The present perfect tense.

   1. ( You / ever / be ) to United State? - No, but I ( be ) England and France.

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   2. When we were in Canada, we ( go ) skiing almost everyday.

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   3. When ( you / meet ) your wife? - 3 years ago.

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    4. Peter ( not phone ) yet. I'm still waiting.

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    5. I ( know ) Helen since we ( be ) at school together.

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     6. My uncle ( live ) in Berlin in 1999.

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      7. We ( not see ) Beth and Mike's house yesterday morning.

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      8. Do you like this picture? My uncle ( paint ) it.

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      9. Mr.Dass ( leave ) for New York late last night.

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      10. Mr and Mrs Grey ( teach ) Rranch here for over 5 years.

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