Final answer:
The variable that the energy and speed of light equations have in common is frequency, represented by the Greek letter nu (ν).
Step-by-step explanation:
The energy and speed of light equations have the variable frequency in common. The energy equation for a photon can be represented as E = hν, where E represents energy, h represents Planck's constant, and ν (nu) represents the frequency of the photon. On the other hand, the speed of light equation is c = λν, where c is the speed of light, λ (lambda) is the wavelength, and ν is also the frequency. The constant c always stands for the speed of light, which is approximately 3.00×108 m/s in a vacuum. Both equations highlight the importance of frequency in understanding the properties of light and energy.