Answer:
The probability that a randomly chosen light bulb will last less than 900 hours is 0.1587.
Step-by-step explanation:
The life span of these light bulbs is normally distributed with a mean of 1000 hours and a standard deviation of 100 hours
Mean = 
Standard deviation =
We are supposed to find the probability that a randomly chosen light bulb will last less than 900 hours.i.e. P(x<900)
So, 

Z=-1
P(x<900)=P(z<-1)=0.1587
Hence the probability that a randomly chosen light bulb will last less than 900 hours is 0.1587.
Recall that sin²Ф+cos²Ф=1,
so cos²Ф=1-sin²Ф=(1+sinФ)(1-sinФ)
the (1-sinФ) in the numerator cancel with the (1-sinФ) in the denominator, the result is 1+sinФ
I'm sorry sorry don't no understand brother
9/10 is the only was you can write it as a fraction because it's 9tenths