因式分解：①x²81；②16-x²；③x²-25y²；④9x²-16y²

因式分解：①x²81；②16-x²；③x²-25y²；④9x²-16y²
因式分解：①x²81；②16-x²；③x²-25y²；④9x²-16y²

因式分解：①x²81；②16-x²；③x²-25y²；④9x²-16y²
解
x²-81
=x²-9²
=(x-9)(x+9)
16-x²
=4²-x²
=(4-x)(4+x)
x²-25y²
=x²-(5y)²
=(x+5y)(x-5y)
9x²-16y²
=(3x)²-(4y)²
=(3x-4y)(3x+4y)

①x²-81=(x+9)(x-9)
②16-x²=(4+x)(4-x)
③x²-25y²=(x+5y)(x-5y)
④9x²-16y²=(3x+4y)(3x-4y)