Question

Rút gọn rồi tính giá trị của biểu thức
a) (x - 10)^2 - x (x + 80) khi x = 0,98
b) (2x + 9)^2 - x (4x + 31) khi x = -16,2
c)4x^2 - 28x + 49 khi x = 4

Answer 1

\(a,=x^2-20x+100-x^2-80x\\ =-100x+100\\ =-100.0,98+100=2\\ b,=4x^2+36x+81-4x^2-31x\\ =5x+81\\ =5.\left(-16,2\right)+81=0\\ c,=\left(2x-7\right)^2\\ =\left(2.4-7\right)^2\\ =1^2=1\)

Answer 2

a) \(\left(x-10\right)^2-x\left(x+80\right)=x^2-20x+100-x^2-80x=100-100x\)

Khi \(x=0,98\) ta có \(100-100x=100-98=2\)

b) \(\left(2x+9\right)^2-x\left(4x+31\right)=4x^2+36x+81-4x^2-31x=5x+81\)

Khi \(x=-16,2\) ta có \(5x+81=5.\left(-16,2\right)+81=0\)

c) \(4x^2-28x+49=\left(2x-7\right)^2\)

Khi \(x=4\) ta có \(\left(2x-7\right)^2=1^2=1\)