Bài 4:
`a)-4x²+9y²`
`=9y²-4x²`
`=(3y)²-(2x)²`
`=(3y+2x)(3y-2x)`
`b)8+(4x-3)³`
`=2³+(4x-3)³`
`=(2+4x-3)[2²-2(4x-3)+(4x-3)²]`
`=(4x-1)(4-8x+6+16x²-24x+9)`
`=(4x-1)(16x²-32x+19)`
`c)(x+1)³-(2-x)³`
`=[(x+1)-(2-x)][(x+1)²+(x+1)(2-x)+(2-x)²]`
`=(x+1-2+x)(x²+2x+1+2x-x²+2-x+4-4x+x²)`
`=(2x-1)[(x²-x²+x²)+(2x+2x-x-4x)+(1+2+4)]`
`=(2x-1)(x²-x+7)`
`d)81-(9-x²)^2`
`=9²-(9-x²)^2`
`=(9+9-x)(9-9+x)`
`=x(18-x)`
Bài 5:
`a)9x²+30x+25`
`=(3x)²+2.3x.5+5²`
`=(3x+5)²`
`4/9x^4-16x²`
`=(2/3x²)^2-(4x)²`
`=(2/3x²+4x)(2/3x²-4x)`
`b)12/5x²y²-9x^4-4/25y^4`
`=-9x^4+12/5x²y²-4/25y^4`
`=-(9x^4-12/5x²y²+4/25y^4)`
`=-[(3x²)^2-2.3x². 2/5y²+(2/5y²)^2]`
`=-(3x²-2/5y²)^2`
`c)a²y²+b²x²-2axby`
`=a²y²-2axby+b²x²`
`=(ay)²-2.ay.bx+(bx)²`
`=(ay-bx)²`
`d)64x²-(8a+b)²`
`=(8x)²-(8a+b)²`
`=(8x+8a+b)(8x-8a-b)`
`e)100-(3x-y)²`
`=10²-(3x-y)²`
`=(10+3x-y)(10-3x+y)`
`g)27x³-a³b³`
`=(3x)³-(ab)³`
`=(3x-ab)[(3x)²+3x.ab+(ab)²]`
`=(3x-ab)(9x²+3xab+a²b²)`
Bài 6:
`a)25²-15²`
`=(25+15)(25-15)`
`=40.10`
`=400`
`b)` Sửa đề:`205^5-95²`
`→205²-95²`
`=(205+95)(205-95)`
`=300.110`
`=33 000`
`c)36²-14²`
`=(36+14)(36-14)`
`=50.22`
`=1100`
`d)950²-850²`
`=(950+850)(950-850)`
`=1800.100`
`=1800 00`
`e)1,24²-2,48.0,24+0,24²`
`=1,24²-2.1,24.0,24+0,24²`
`=(1,24-0,24)²`
`=1²`
`=1`
