# Thread: Help w/ Bernoulli Trials

1. ## Help w/ Bernoulli Trials

"Solve the probabilities for the following Bernoulli Trials where p = pr[Success] and q = pr[Failure]:
(1) At least 3 successes in 9 trials with p = 0.7

(2) At least 3 failures in 10 trials with p = 0.35"

This one has thrown me off a bit. I know the formula: C(n,r)(p^r)(q^(n-r)) but there is another one that I think I am supposed to use that I don't fully understand. The one that I don't quite grasp is: 1-C(n,r) (p^r)...something something that's where I lose it. If I can see this one done once through I should have the rest of it down.

Thanks in advance for any help,
Corwin

2. at least 3 success means:

$P(X \geq 3) = 1-P(X < 3)= 1-[P(X=0)+P(X=1)+P(X=2)]$

3. Originally Posted by harish21
at least 3 success means:

$P(X \geq 3) = 1-P(X < 2)= 1-[P(X=0)+P(X=1)]$
Got it, thanks.

4. The webwork I'm doing for class still says I'm doing it wrong. Can someone show me how it looks when it's all plugged into the equation? If someone could just do the first one I could go from there.

5. Originally Posted by Corwin
The webwork I'm doing for class still says I'm doing it wrong. Can someone show me how it looks when it's all plugged into the equation? If someone could just do the first one I could go from there.
We do not know what notation your webwork expects.

It maybe something like: $\sum\limits_{k = 3}^9 {C(9,k)p^k q^{9 - k} }$ or even $1-\sum\limits_{k = 0}^2 {C(9,k)p^k q^{9 - k} }$.

Notation is the greatest drawback to teaching mathematics on the web.