Question

a/Cho hàm số y = f(x) = 2x³ - 3x .Tính f(-4) ; f(-2) ; f(0) ; f(\(\dfrac{2}{3}\))

b/Cho hàm số y = f(x) = x³ - 2. Tìm x biết f(x) = 25

Answer 1

a,\(f\left(-4\right)=2.\left(-4\right)^3-3.\left(-4\right)=2.\left(-64\right)+12=-128+12=-116\)

\(f\left(-2\right)=2.\left(-2\right)^3-3.\left(-2\right)=2.\left(-8\right)+6=-16+6=-10\)

\(f\left(0\right)=2.0^3-3.0=2.0-0=0-0=0\)

\(f\left(\dfrac{2}{3}\right)=2.\left(\dfrac{2}{3}\right)^3-3.\left(\dfrac{2}{3}\right)=2.\dfrac{8}{27}-2=\dfrac{16}{27}-2=\dfrac{-38}{27}\)

b,

\(f\left(x\right)=25\rightarrow y=25\)

Ta có : \(x^3-2=25\)

\(\rightarrow x^3=27\)

\(\Rightarrow x=3\) ( Vì 27 = \(3^3\) )