use 
to get 
then use
and 
also, 
to get 
then again use the first identity In both pairs, i.e.

to get 
multiply and divide by 4 to get the RHS.
because, 
squaring both sides, 
Answer:

Explanation:
You need to find the probability that exactly three of the first 11 inspected packages are damaged and the fourth is damaged too.
<u>1. Start with the first 11 inspected packages:</u>
a) The number of combinations in which 11 packages can be taken from the 20 available packages is given by the combinatory formula:


b) The number of combinations in which 3 damaged packages can be chossen from 7 damaged packages is:

c) The number of cominations in which 8 good packages can be choosen from 13 good pacakes is:

d) The number of cominations in which 3 damaged packages and 8 good packages are chosen in the first 11 selections is:

e) The probability is the number of favorable outcomes divided by the number of possible outcomes, then that is:

Subsituting:


<u>2. The 12th package</u>
The probability 12th package is damaged too is 7 - 3 = 4, out of 20 - 11 = 9:
<u>3. Finally</u>
The probability that exactly 12 packages are inspected to find exactly 4 damaged packages is the product of the two calculated probabilities:

Answer:
Probability to solve 7 problem will be equal to
Step-by-step explanation:
It is given total number of question = 10
She can solve 8 of them from 10 questions
We have to find the probability that she can solve 7 question
Probability will be equal to ratio of favorable outcome to total sample space
Favorable outcome will be equal to 
Total sample space 
So probability will be equal to 
So probability to solve 7 problem will be equal to
Its 108 because the triangle is acute so all angle are less than or equal to 90 and i did 54+54+72=180 ant the measurement of the exterior angle is 108