Answer:
and 
Step-by-step explanation:
Recall that 
is a value of the function cosine for the special angles: 
and 
, then:
Let's try plugging in some negative numbers. Let's do x=-1. 5+-1=4. So we know that if we put in a negative number for x, then n will be positive. But what if we do a number greater than -5, because 5+-5=0. So let's try x=-6. So 5+(-6)=-1. Hmm. So here it is. We know that any number under -5 will be positive and any number above -5 will be negative.
Answer:
an = a(n-1) - 7
Step-by-step explanation:
the second term is smaller than the first term 7.
a20 = a19 - 7
a19 = a18 - 7
=> a20 = (a18 - 7) - 7
Then you can accumalate until a1
-> a20 = a1 - 19*7 = 86 - 19*7 = -47
Imposible triangle, lagest side must be > sum other sides.
