y=x3y = x^3

Graph of the Cubic Function y=x3y = x^3

y=x3y = x^3 is the third-degree monomial obtained by multiplying xx by itself three times1. It flattens out for an instant at the origin, then climbs without bound to the upper right.

Domain and range

  • Both the domain and the range are all real numbers
  • The function increases monotonically
  • Odd function
  • The origin is an inflection point

Because the exponent is odd, a negative xx gives a negative yy: unlike x2x^2, this function takes negative values.

Symmetry

From (x)3=x3(-x)^3 = -x^3, it is an odd function with point symmetry about the origin. Only the sign flips, so x=2x = 2 gives 88 while x=2x = -2 gives 8-8. The graph lies in the first and third quadrants.

Monotonicity and inflection

The derivative is y=3x2y' = 3x^2, positive everywhere except at x=0x = 0, so the function increases monotonically over the whole line. At x=0x = 0 the derivative vanishes and the tangent line lies along the xx-axis, but the function keeps increasing on both sides, so this is neither a maximum nor a minimum.

The second derivative y=6xy'' = 6x changes sign at x=0x = 0, which makes the origin an inflection point: the curve is concave down for x<0x < 0 and concave up for x>0x > 0.

Compared with the square

xxx2x^2x3x^3
0.50.50.250.250.1250.125
111111
224488
2-2448-8

The curve flattens near the origin and shoots up once x|x| exceeds 11. Where x2x^2 is even and never negative, x3x^3 is odd and preserves sign.

Relationships with other functions

Since x3x^3 is a one-to-one map from the reals onto the reals, it has an inverse: the cube root y=x3y = \sqrt[3]{x}. The equation x3=ax^3 = a has exactly one real solution for every real aa.

It is the archetype of the odd powers x5x^5, x7x^7 and beyond. Every odd power passes through (1,1)(-1, -1), (0,0)(0, 0) and (1,1)(1, 1).

Applications

Doubling the side of a cube multiplies its volume by 88: the volumes of similar solids grow with the cube of length.

  • Volumes of similar solids
  • Cardano's formula for solving cubic equations
  • Cubic splines and Bezier curves in graphics and design2
  1. Cubic function, Wikipedia
  2. Bezier curve, Wikipedia