将3x²-10x+7用求根公式法因式分解,还有一题4x²+8x-21
1个回答

(1)

令3x^2-10x+7=0,△=10^2-4×3×7=16

则x=(10±4)/6

x=7/3或x=1

所以3x^2-10x+7=3(x-7/3)(x-1）=(3x-7)(x-1)

(2)

4x^2+8x-21=0,△=8^2-4×4×(-21)=400

则x=(-8±20)/8

x=-7/2或x=3/2

4x^2+8x-21=4(x+7/2)(x-3/2)=(2x+7)(2x-3)

(3)

3x^2+4xy-y^2

=3(x^2+(4/3)xy+(4/9)y^2)-(4/3)y^2-y^2

=3(x+(2/3)y)^2-(7/3)y^2

=3[(x+(2/3)y)^2-(7/9)y^2]

=3(x+(2/3)y+(√7/3)y)(x+(2/3)y-(√7/3)y)

=3(x+((2+√7)/3)y)(x+((2-√7)/3)y)

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