The above formulae are academically accurate if you need a computer algorithm or similar.
However, if you just want a reasonably accurate pilot's rule of thumb for use in the air:
TAS = CAS (or RAS) x (1.75% per thousand feet)
So, if you are at, say, 10,000 feet with a CAS of 100 knots, ten lots of 1.75% is 17.5 %, so your TAS is 117.5 knots.
Provided the temperature is fairly close to ISA, it's very accurate up to about 25,000 feet and not too bad even higher.