45 is the ninth triangular number; it is the sum the first nine integers: 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45.
45 is the only number that is the sum of its digits multiplied by 5.
If the first five powers of two are each subtracted from 45, all the results are prime: 45 – 2 = 43; 45 – 4 = 41; 45 – 8 = 37; 45 – 16 = 29; 45 – 32 = 11.
The chemical element rhodium has atomic number 45.
Source: Number Gossip
