Easy way to square numbers ending in 5

Here is the formula then I will show examples:

x(x+1)*100+25

 So you always know it will end in 25 so all you have to do is figure out what goes in front.  Which is pretty easy.  You multiply the number to the left of the 5 by the next number above it.  That's it.  Here are a few examples:

15 = (1*2)25 = 225

25 = (2*3)25 = 625

35 = (3*4)25 = 1225

45 = (4*5)25 = 2025

105 = (10*11)25 = 11025