1. a) 2x ( x2 - 7x - 3 )
= 2x. x2 - 2x.7x - 2x. 3
= 2.x3 - 14x2 - 6x
b)\(\left(-2x^3+\dfrac{3}{4}y^2-7xy\right).4xy^2\)
= \(\left(-2x^3\right)4xy^2+\dfrac{3}{4}y^2.4xy^2-7xy.4xy^2\)
= \(-8x^4y^2+3xy^4-28x^2y^3\)
c)\(\left(-5x^3\right)\left(2x^2+3x-5\right)\)
= \(\left(-5x^3\right)2x^2+\left(-5x^3\right)3x-\left(-5x^3\right)5\)
=\(-10x^5-15x^4+25x^3\)
d)\(\left(2x^2-\dfrac{1}{3}xy+y^2\right)\left(-3x^3\right)\)
= \(2x^2\left(-3x^3\right)-\dfrac{1}{3}xy\left(-3x^3\right)+y^2\left(-3x^3\right)\)
= \(-6x^5+x^4y-3x^2y^3\)
2.a)\(A=x^2\left(x+y\right)-y\left(x^2-y\right)+2017\) với x = 1; y = -1
= \(x^3+x^2y-x^2y+y^2+2017\)
= \(x^3+y^2+2017\)
Tại x = 1; y = -1 , ta có:
A = \(1^3+\left(-1\right)^2+2017\)
= 2019
Vậy A = 2019
b) \(B=5x\left(x-4y\right)-4y\left(y-5x\right)-\dfrac{11}{20}\) với x = 0,6 ; y = -0,75
= \(5x^2-20xy-4y^2+20xy-\dfrac{11}{20}\)
= \(5x^2-4y^2-\dfrac{11}{20}\)
Tại x = 0,6 ; y = -0,75, ta có:
B = \(5\left(0,6\right)^2-4\left(-0,75\right)^2-\dfrac{11}{20}\)
= -1
Vậy B= -1
