The function f(t) = 4 cos(pi over 3t) + 15 represents the tide in Bright Sea. It has a maximum of 19 feet when time (t) is 0 and a minimum of 11 feet. The sea repeats this cycle every 6 hours. After five hours, how high is the tide?

13.5 feet

16 feet

18.5 feet

17 feet

Question

The function f(t) = 4 cos(pi over 3t) + 15 represents the tide in Bright Sea. It has a maximum of 19 feet when time (t) is 0 and a minimum of 11 feet. The sea repeats this cycle every 6 hours. After five hours, how high is the tide?

13.5 feet

 16 feet

 18.5 feet

 17 feet

vgonzalez · Answer

@elementwielder

campbell_st · Answer

well substitute t = 5 and see what happens

vgonzalez · Answer

how would you do that plug in the numbers into formula?

anonymous · Answer

sorry idk

vgonzalez · Answer

okay so would i did cos(pie/3 x 5) and got 0.999

campbell_st · Answer

well its 

$$f(5) = 4 \times \cos(\frac{\pi}{3} \times 5) + 15$$

is your calculator set for radians or degrees...?

vgonzalez · Answer

i think radians. not sure. I'm trying to solve the pie/3 x 5) first and got a different answer.. pie/243.

campbell_st · Answer

well try (5 times pi)/3

vgonzalez · Answer

5.23?

campbell_st · Answer

no you're not trying to solve it... you just need to evaluate it... when t = 5

campbell_st · Answer

ok... so work in degrees..... pi/3 = 60

so use 

$$f(5) = 4\times \cos(60 \times 5) + 15$$

vgonzalez · Answer

okay so far i did the 60 x 5 = 300
Now i just need to find cosine next correct?

campbell_st · Answer

yep... thats right

campbell_st · Answer

makes sense to me... and if you had the calculator in radians, you would have the same answer