The triangle symbol is read as delta. It means "change". If you encounter this symbol , you will have to subtract the initial condition from the final condition.
For example, water at 25 degrees C is heated to 100 degrees C, what is delta T
delta T = T2 - T1 = 100 - 25 = 75 degrees C
the temperature of water is increased by 75 degrees C from 25 to 100 degres C
Δ = delta
