`25-a^2+2ab-b^2`

`=25-(a^2-2ab+b^2)`

`=5^2-(a-b)^2`

`=(5+a-b)(5-a+b)`

`2^(x+1)-2^x=32`

`2^x . 2 -2^x =32`

`2^x (2-1) =32`

`2^x=32`

`2^x=2^5`

`x=5`.

C.

3. B
4. D
5.  A
6. C
6. C
8. D

PTK = 64+32+16.4=160 (đvC)

`3^1 .3^n + 5.3^(n+1)=16`

 `3^(n+1) + 5.3^(n+1) =16`

`3^(n+1) . (1+5) =16`

`3^n . 3^1 = 8/3`

`3^n = 8/9`

`->` Không có n thỏa mãn.

a) `16-24x+9x^2-25y^2`
`=[4^2-2.4.3x+(3x)^2]-25y^2`
`=(4-3x)^2-(5y)^2`
`=(4-3x+5y)(4-3x-5y)`
b) `x^2-5xy-36y^2`
`=x^2-9xy+4xy-36y^2`
`=x(x-9y)+4y(x-9y)`
`=(x-9y)(x+4y)`

Mai said the stress of the entrance examinations had made her physically ill.

13. A
14. D
15. A
16. C
17. B
18. A
19. B
20. A
21. D

misses