Given:
cos 120°
To find:
The exact value of cos 120° in simplest form with a rational denominator.
Solution:
We have,

It can be written as

![[\because \cos (90^\circ-\theta)=-\sin \theta]](https://tex.z-dn.net/?f=%5B%5Cbecause%20%5Ccos%20%2890%5E%5Ccirc-%5Ctheta%29%3D-%5Csin%20%5Ctheta%5D)
![[\because \sin 30^\circ=\dfrac{1}{2}]](https://tex.z-dn.net/?f=%5B%5Cbecause%20%5Csin%2030%5E%5Ccirc%3D%5Cdfrac%7B1%7D%7B2%7D%5D)

Therefore, the exact value of cos 120° is
.
ANSWER: The size of a red blood cell IS approximately half the size of a plant cell.
EXPLANATION:
<em>1. We divide the size of the plant cell by 2 to see if half of its size is the size of a red blood cell. </em>

Therefore, half of a plant cell's size is 0.000006325 m.
<em>2. We now compare the size of half of the plant cell to the size of the red blood cell.</em>
0.000006325 m and 0.000007 m are very close in size; therefore, the size of a red blood cell IS approximately half of a plant cell.
2 Letters chosen among 6 with no repeat:
a) the first has 6 ways
the second has 5 ways
Then to choose 2 letters among 6, with no repeat, there are 6x5 = 30 ways
b) 3 numbers are taken from the digits 0 through 9 with repeats (total = 10)
the first has 10 ways
the second has 10 ways
the third has 10 ways
Then to choose 3 numbers among 10, with repeat, there are 10x10x10= 1000 ways
And the total serial numbers that can be generated is
30 x 1000 = 3000