
exponential functions
A man jumps from above  his speed of descent is given by V = 50(12^0.2t) m/s where t is the time in seconds. Find the time taken for his speed to reach 40 m/s.
I know to set the equation to:
40 = 50(1  2^0.2t)
but i can't seem to carry it out from there..because i can't take the log of both sides when the numbers are negative.

rearrange it to get the $\displaystyle 2^{0.2t}$ by itself.. eg:
$\displaystyle 40 = 50(12^{0.2t})$
=>
$\displaystyle 40 = 50  50*2^{0.2t}$
=>
$\displaystyle 10 = 50*2^{0.2t}$
=>
$\displaystyle 1/5 = 2^{0.2t}$
then take logs of both side and continue. you should get something like $\displaystyle t = 5 \frac{\log{5}}{\log{2}}$