25² - 15²

= (25 - 15)(25 + 15)

= 10 . 40

= 400

205² - 95²

= (205 - 95)(205 + 95)

= 110 . 300

= 33 000

36² - 14²

= (36 - 14)(36 + 14)

= 22 . 50

= 1 100

950² - 850²

= (950 - 850)(950 + 850)

= 100 . 1 800

= 180 000

a) 25² - 15²

= (25 - 15).(25 + 15)

= 10.40

= 400

b) 36² - 14²

= (36 + 14).(36 - 14)

= 50.22

= 1100

d) 950² - 850²

= (950 + 850).(950 - 850)

= 1 800.100

= 180 000

Tất cả đều dùng HĐT đáng nhớ (3) nha!(có trong SGK 8-tập 1)

Mk chỉ làm câu a thôi nha

\(a;25^2-15^2=\left(25-15\right)\left(25+15\right)\)

Bài 1: 

a: \(\left(3x+2\right)^2-4=3x\left(3x+4\right)\)

Bài 2: 

a: \(4x^2+4x+1\)

b: \(9x^2+9x+\dfrac{9}{4}\)

5:

a: (2x-5)(2x+5)=4x^2-25

b: (3x-5y)(3x+5y)=9x^2-25y^2

c: (3x+7y)(3x-7y)=9x^2-49y^2

d: (2x-1)(2x+1)=4x^2-1

4:

a: 2003*2005=(2004-1)(2004+1)=2004^2-1<2004^2

b: 8(7^2+1)(7^4+1)(7^8+1)

=1/6*(7-1)(7+1)(7^2+1)(7^4+1)(7^8+1)

=1/6(7^2-1)(7^2+1)(7^4+1)(7^8+1)

=1/6(7^16-1)<7^16-1

5:

a: (2x-5)(2x+5)=4x^2-25

b: (3x-5y)(3x+5y)=9x^2-25y^2

c: (3x+7y)(3x-7y)=9x^2-49y^2

d: (2x-1)(2x+1)=4x^2-1

mik chỉ biết bài 5 thôi !

Bài 9:

a) Ta có: \(A=\left(2x+y\right)^2-\left(2x+y\right)\left(2x-y\right)+y\left(x-y\right)\)

\(=4x^2+4xy+y^2-4x^2+y^2-xy-y^2\)

\(=3xy-y^2\)

\(=3\cdot\left(-2\right)\cdot3-3^2=-18-9=-27\)

b) Ta có: \(B=\left(a-3b\right)^2-\left(a+3b\right)^2-\left(a-1\right)\left(b-2\right)\)

\(=a^2-6ab+9b^2-a^2-6ab-9b^2-ab+2a+b-2\)

\(=-13ab+2a+b-2\)

\(=-13\cdot\dfrac{1}{2}\cdot\left(-3\right)+2\cdot\dfrac{1}{2}+\left(-3\right)-2\)

\(=\dfrac{31}{2}\)

Bài 7: 

a) \(498^2=\left(500-2\right)^2=250000-2000+4=248004\)

b) \(93\cdot107=100^2-7^2=10000-49=9951\)

c) \(163^2+74\cdot163+37^2=\left(163+37\right)^2=200^2=40000\)

d) \(1995^2-1994\cdot1996=1995^2-1995^2+1=1\)

e) \(9^8\cdot2^8-\left(18^4-1\right)\left(18^4+1\right)\)

\(=18^8-18^8+1=1\)

f) \(125^2-2\cdot125\cdot25+25^2=\left(125-25\right)^2=100^2=10000\)

Bài 8:

a) Ta có: \(\left(x^2+3x+1\right)^2-2\left(x^2+3x+1\right)\left(3x-1\right)+\left(3x-1\right)^2\)

\(=\left(x^2+3x+1-3x+1\right)^2\)

\(=\left(x^2+2\right)^2\)

\(=x^4+4x^2+4\)

b) Ta có: \(\left(3x^3+3x+1\right)\left(3x^3-3x+1\right)-\left(3x^3+1\right)^2\)

\(=\left(3x^3+1\right)^2-9x^2-\left(3x^3+1\right)^2\)

\(=-9x^2\)

c) Ta có: \(\left(2x^2+2x+1\right)\left(2x^2-2x+1\right)-\left(2x^2+1\right)^2\)

\(=\left(2x^2+1\right)^2-4x^2-\left(2x^2+1\right)^2\)

\(=-4x^2\)

Chọn đáp án C bạn nhé

\(205^2-95^2=\)

\(=\left(205-95\right)\left(205+95\right)\)

\(=200.300\)

\(=60000\)

\(36^2-14^2=\)

\(=\left(36-14\right)\left(36+14\right)\)

\(=22.50\)

\(=1100\)

\(205^2-95^2=\left(205-95\right)\left(205+95\right)=110.300=33000\)

\(36^2-14^2=\left(36-14\right)\left(36+14\right)=22.50=1100\)

\(97^2-3^2=\left(97-3\right)\left(97+3\right)=94.100=9400\)

\(205^2-95^2=\left(205+95\right)\left(205-95\right)=300.110=3.100.11.10=33000\)

\(36^2-14^2=\left(36+14\right)\left(36-14\right)=50.22=1100\)

\(97^2-3^2=\left(97+3\right)\left(97-3\right)=100.94=9400\)

\(89^2-18.89+9^2=\left(89-9\right)^2=80^2=6400\)

\(41^2+82.59+59^2=\left(41+59\right)^2=100^2=10000\)

\(1,24^2-2,84.0,24+0,24^2=\left(1,24-0,24\right)^2=1^2=1\)