3(x-y)=9,x^2+y^2=11
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3(x-y)=9,x^2+y^2=11
1.xy的值 2.3x^2-3xy^2
1.xy的值 2.3x^2-3xy^2
∵ 3(x - y)= 9
∴ x - y = 3
∴ (x - y)² = 9
x ² - 2 x y + y ² = 9
∵ x ² + y ² = 11
∴ 11 - 2 x y = 9
- 2 x y = 9 - 11
- 2 x y = - 2
x y = 1
∵ 3 x ² y - 3 x y ²
= 3 x y(x - y)
又∵ x y = 1 ,x - y = 3
∴ 原式 = 3 x y(x - y)
= 3 × 1 × 3
= 9
∴ x - y = 3
∴ (x - y)² = 9
x ² - 2 x y + y ² = 9
∵ x ² + y ² = 11
∴ 11 - 2 x y = 9
- 2 x y = 9 - 11
- 2 x y = - 2
x y = 1
∵ 3 x ² y - 3 x y ²
= 3 x y(x - y)
又∵ x y = 1 ,x - y = 3
∴ 原式 = 3 x y(x - y)
= 3 × 1 × 3
= 9
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