b) 6x - 9 - x²
= - (x² - 6x + 9 )
= - ( x² - 2.x.3 + 3² )
= - ( x - 3 )²
c) x² - 16
= x² - 4²
= ( x - 4 )( x + 4)
d) 9x² - 25
= ( 3x )² - 5²
= ( 3x - 5 )( 3x + 5 )
e ) x⁴ - y⁴
= ( x²)² - ( y² )²
= ( x² - y² )( x² + y² )
f) x⁶ -y⁶
= ( x³ )² - ( y³)^2
=( x³ - y³ )( x³ + y³ )

a) 26² + 52.24 + 24²
= 26² + 2.26.24 + 24²
= ( 26 + 24 )²
= 50² = 2500
b) 3003² - 3^2
= ( 3003 + 3 ) ( 3003 - 3 )
= 3006 . 3000 = 9018000
c) 87² + 73² - 27² -13²
= ( 87² - 13² ) + ( 73² - 27² )
= [ ( 87 + 13 )( 87- 13 )] + [ ( 73 - 27 )( 73 + 27 ) ]
= ( 100 . 74 ) + ( 46 . 100 )
= 7400 + 4600 = 12000
d)79² - 79.58 + 29²
= 79² - 2.79.29 + 29²
= ( 79 - 29 )²
= 50² = 2500

1 That's Bod's, _____?

a. is that b. isn’t that c. are they d. aren’t they

2 My wife had never been to Hue, and _____.

a. never have b. so did we c. we did either d. neither we did

3 They must do as they are told, _____

a. so must I b. I must either c. I do too d. I am too

4 'I don’t like strong coffee' 'No, _____"

a. I don't too b. either don't I c. me either d. neither don't I

5 He hardly has anything nowadays, _____?

a. has he b. doesn’t he c. does she d. hasn't she

1. Sai
2.
a) 9x² - 6x + 1
= ( 3x)² - 2.3x.1 + 1²
= ( 3x - 1 )²
b) ( 2x + 3y )² + 2x ( 2x + 3y ) +1
= ( 2x + 3y + 1 )²

(x-3).(2x+4)+(-x+1).(x-2)
= ( 2x² + 4x - 6x -12 ) + ( -x² + 2x + x - 2 )
= 2x² + 4x - 6x -12 - x² + 2x + x - 2
= x² + x - 14

( x³ - 2x² + x - 1 ) ( 5-x )
= 5x³ - x⁴ - 10x² + 2x³ + 5x - x² - 5 + x
= - x⁴ + 7x³ -11x² + 6x - 5
=> ( x³ - 2x² + x -1 ) ( x - 5 )
= x⁴ - 7x³ + 11x² - 6x + 5

a) [ \(\dfrac{1}{2}\)x - 1 ] ( 2x-3)
= x² - \(\dfrac{3}{2}\)x - 2x + 3

b) ( x- 7 ) ( x - 5 )
= x² - 5x - 7x + 35

c) ( x- \(\dfrac{1}{2}\)) ( x + \(\dfrac{1}{2}\)) ( 4x - 1 )
= ( x² - \(\dfrac{1}{4}\)) ( 4x - 1 )
= 4x³ - x² - x + \(\dfrac{1}{4}\)

B= 2y-x - {2x-y-[y+3x-(5y-x)]}
B= 2y-x- [ 2x-y- ( y + 3x-5y+x)
B= 2y-x-( 2x-y-y-3x+5y-x)
B= 2y-x-2x+y+y+3x-5y+x
B= -y +x

Xét vế trái: ( 100 +a )(100+b)
= 10000 +100b + 100a + ab
Xét vế phải: ( 100 + a +b ).100 + ab
= 10000 + 100a + 100b + ab
= VT
\(\Rightarrow\) đpcm

( 3 + xy² )²
= 3² + 2.3. xy² + ( xy² )²
= 9 + 6xy² + x²y⁴