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九年级数学解答题一般
题目
解下列一元二次方程:
(1)2x23=8x(配方法)(1)2x^{2}-3=8x(配方法).
(2)5x2=42x(2)5x^{2}=4-2x.
(3)2y2y=2(12y)(3)2y^{2}-y=2\left(1-2y\right).
知识点:二次根式、二元一次方程组、二元一次方程组的解、二元一次方程组的应用、非负数的性质:偶次方章节:未标注

答案与解析

答案

(1)2x23=8x\left(1\right)2x^{2}-3=8x
2x28x=32x^{2}-8x=3
x24x=32{x}^{2}-4x=\frac{3}{2}
x24x+4=32+4{x}^{2}-4x+4=\frac{3}{2}+4
(x2)2=112{(x-2)}^{2}=\frac{11}{2}
x2=±222x-2=±\frac{\sqrt{22}}{2}
x1=2+222x2=2222\therefore {x}_{1}=2+\frac{\sqrt{22}}{2},{x}_{2}=2-\frac{\sqrt{22}}{2}
(2)5x2=42x(2)5x^{2}=4-2x
5x2+2x4=05x^{2}+2x-4=0
a=5a=5b=2b=2c=4c=-4
b24ac=224×5×(4)=84>0b^{2}-4ac=2^{2}-4\times 5\times \left(-4\right)=84 \gt 0
x=2±842×5\therefore x=\frac{-2±\sqrt{84}}{2×5}
x1=1+215x2=1215\therefore {x}_{1}=\frac{-1+\sqrt{21}}{5},{x}_{2}=\frac{-1-\sqrt{21}}{5}
(3)2y2y=2(12y)(3)2y^{2}-y=2\left(1-2y\right)
y(2y1)+2(2y1)=0y\left(2y-1\right)+2\left(2y-1\right)=0
(2y1)(2+y)=0(2y-1)\left(2+y\right)=0
2y1=02y-1=02+y=02+y=0
y1=12y2=2\therefore {y}_{1}=\frac{1}{2},{y}_{2}=-2.

解析

(1)2x23=8x\left(1\right)2x^{2}-3=8x
2x28x=32x^{2}-8x=3
x24x=32{x}^{2}-4x=\frac{3}{2}
x24x+4=32+4{x}^{2}-4x+4=\frac{3}{2}+4
(x2)2=112{(x-2)}^{2}=\frac{11}{2}
x2=±222x-2=±\frac{\sqrt{22}}{2}
x1=2+222x2=2222\therefore {x}_{1}=2+\frac{\sqrt{22}}{2},{x}_{2}=2-\frac{\sqrt{22}}{2}
(2)5x2=42x(2)5x^{2}=4-2x
5x2+2x4=05x^{2}+2x-4=0
a=5a=5b=2b=2c=4c=-4
b24ac=224×5×(4)=84>0b^{2}-4ac=2^{2}-4\times 5\times \left(-4\right)=84 \gt 0
x=2±842×5\therefore x=\frac{-2±\sqrt{84}}{2×5}
x1=1+215x2=1215\therefore {x}_{1}=\frac{-1+\sqrt{21}}{5},{x}_{2}=\frac{-1-\sqrt{21}}{5}
(3)2y2y=2(12y)(3)2y^{2}-y=2\left(1-2y\right)
y(2y1)+2(2y1)=0y\left(2y-1\right)+2\left(2y-1\right)=0
(2y1)(2+y)=0(2y-1)\left(2+y\right)=0
2y1=02y-1=02+y=02+y=0
y1=12y2=2\therefore {y}_{1}=\frac{1}{2},{y}_{2}=-2.

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