Question

What does a triangle written above the arrow in an equation
represent?

Answer 1

If the reaction requires energy, it is often indicated above the arrow. A capital Greek letter delta (Δ) is written on top of the reaction arrow to show that energy in the form of heat is added to the reaction; hv is written if the energy is added in the form of light.

Answer 2

it means that to get the product, the reactants were heated