Fact about logarithms:

log_a x = (log_b x) * (log_a b)

So when you want to change the base of the log from b to a,
you multiply by (log_a b).

Why is this true?

* By definition of log, x = b raised to the power (log_b x) 
* Then take the log_a of both sides:
   log_a x = log_a (b^{log_b x})
* By rules of logarithms, the righthand side becomes (log_b x) * (log_a b)
* Thus log_a x = (log_b x) * (log_a b)

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Now for the exercise:  Show log_10 (x^2 + 1) = Theta(log_2 x).

(log_10 2) * (log_2 x) = log_10 x      by rule above
      <= log_10 (x^2 + 1)            
      =  log_2 (x^2 + 1) * (log_10 2)  by rule above 
      <= log_2 (2*x^2) * (log_10 2)
      = (1 + 2*log_2 x) * (log_10 2)   by rules of logs
      <= 3*(log_2 x) * (log_10 2)
      = 3(log_10 2) * (log_2 x).

So we can let C_1 = (log_10 x), C_2 = 3(log_10 2), and k = 2.