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a)B=3x3 -2y3-6x2y2+xy
B=(3x3-6x2y2)+(xy-2y3)
B=3x2(x-2y2)+y(x-2y2)
B=(x-2y2)(3x2+y)
tại x=\(\frac{2}{3}\)và y=\(\frac{1}{2}\)ta có B=(x-2y2)(3x2+y)=(\(\frac{2}{3}\)-2*\(\frac{1}{2}\)^2 )(3*\(\frac{2}{3}\)^2+\(\frac{1}{2}\))=\(\frac{1}{6}\)*\(\frac{11}{6}\)=\(\frac{11}{36}\)
b)C= 2x+xy2-x2y-2y
C=(2x-2y)+(xy2-x2y)
C=2(x-y)-xy(x-y)
C=(2-xy)(x-y)
tại x=\(-\frac{1}{2}\)và y=\(-\frac{1}{3}\)ta có C=(2-xy)(x-y)=(2-\(-\frac{1}{2}\)*\(-\frac{1}{3}\))(\(-\frac{1}{2}\)+\(\frac{1}{3}\))=\(\frac{-11}{36}\)
+) \(A=x^2-y+xy^2-x\)
\(A=\left(x^2-y\right)+\left(xy^2-x\right)\)
\(A=\left(x^2-y\right)+x\left(y^2-1\right)\)
Tại x = -5, y = 2 ta có :
\(A=\left[\left(-5\right)^2-2\right]+\left(-5\right)\left(2^2-1\right)=8\)
+) \(B=3x^3-2y^3-6x^2y^2\)
\(B=3x^3-\left(2y^3+6x^2y^2\right)=3x^3-2y^2\left(y+3x^2\right)\)
Tại x = 2/3, y = 1/2 ta có :
\(B=3.\left(\dfrac{2}{3}\right)^3-2.\left(\dfrac{1}{2}\right)^2.\left(\dfrac{1}{2}+3.\dfrac{4}{9}\right)=\dfrac{55}{36}\)
+) \(C=2x+xy^2-x^2y-y\)
\(C=\left(2x+xy^2\right)-\left(x^2y+y\right)=x\left(2+y^2\right)-y\left(x^2+1\right)\)
Tại x= -1/2, y = -1/3 ta có :
\(C=\left(\dfrac{-1}{2}\right)\left[2+\left(\dfrac{-1}{3}\right)^2\right]-\left(-\dfrac{1}{3}\right)\left[\left(\dfrac{-1}{2}\right)^2+1\right]=\left(-\dfrac{19}{18}\right)-\left(-\dfrac{5}{12}\right)=\dfrac{-23}{36}\)
Ta có: \(3x^3-2y^3-6x^2y^2+xy\)
\(=\left(3x^3-6x^2y^2\right)+\left(xy-2y^3\right)\)
\(=3x^2\left(x-2y^2\right)+y\left(x-2y^2\right)\)
\(=\left(x-2y^2\right)\left(3x^2+y\right)\)
\(=\left(\dfrac{2}{3}-2\cdot\dfrac{1}{4}\right)\cdot\left(3\cdot\dfrac{4}{9}+\dfrac{1}{2}\right)\)
\(=\left(\dfrac{2}{3}-\dfrac{1}{2}\right)\cdot\left(\dfrac{4}{3}+\dfrac{1}{2}\right)\)
\(=\dfrac{1}{6}\cdot\dfrac{11}{6}=\dfrac{11}{36}\)
a) \(A=x^2y+y+xy^2-x\) (hẳn đề là vậy)
\(A=xy\left(x+y\right)+\left(y-x\right)\)
\(A=\left(-5\right).2\left(-5+2\right)+2+5\)
\(A=30+7=37\)
b) \(B=3x^3-2y^3-6x^2y^2+xy\)
\(B=3.\left(\frac{2}{3}\right)^3-2.\left(\frac{1}{2}\right)^3-6.\left(\frac{2}{3}\right)^2.\left(\frac{1}{2}\right)^2+\frac{2}{3}.\frac{1}{2}\)
\(B=\frac{8}{9}-\frac{1}{4}-\frac{2}{3}+\frac{1}{3}\)
\(B=\frac{11}{36}\)
c) \(C=2x+xy^2-x^2y-2y\)
\(C=2.\left(-\frac{1}{2}\right)+\left(-\frac{1}{2}\right).\left(-\frac{1}{3}\right)^2-\left(-\frac{1}{2}\right)^2.\left(-\frac{1}{3}\right)-2.\left(-\frac{1}{3}\right)\)
\(C=-1-\frac{1}{18}+\frac{1}{12}+\frac{2}{3}\)
\(C=-\frac{11}{36}\)