Bài 7:
a,(2x+1)²-4(x+2)²=9
⇔4x²+4x+1-4x²-16x-16=9
⇔-12x=24
⇔x=-2
Vậy x=-2
b,(x+3)²-(x-4)(x+8)=1
⇔x²+6x+9-x²-8x+4x+32=1
⇔2x=-40
⇔x=-20
Vậy x=-20
c,3(x+2)²+(2x-1)²-7(x+3)(x-3)=36
⇔3x²+12x+12+4x²-4x+1-7(x²-9)=36
⇔3x²+12x+12+4x²-4x+1-7x²+63=36
⇔8x=-40
⇔x=-5
Vậy x=-5
Bài 8:
a,19²=(20−1)²=202−2.20.1+12=400−40+1=361
28²=(30−2)²=302−2.30.2+22=900−120+4=784
81²=(80+1)²=802+2.80.1+12=6400+160+1=6561
91²=(90+1)²=902+2.90.1+12=8100+180+1=8281
b,19.21=(20−1).(20+1)=202−12=400−1=399
29.31=(30−1).(30+1)=302−12=900−1=899
39.41=(40−1).(40+1)=402−11=1600−1=1599
c,29²-8²=(29-8)(29+8)=21.37=777
56²-46²=(56-46)(56+46)=10.102=1020
67²-56²=(67-56)(67+56)=11.123=1353
