Trig question in the form of a recurrence equation.

S

Sonal7

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I just dont understand the last line of the answer to parts a and b. Where did the minus sign come from ? I plotted these on Geogbra and still clueless.
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lev888 said:
Note how the argument changed with the sign change.
Click to expand...
Yes i noticed that. cos pi is always -1, but i am not sure about the value of n as its a variable. So if n were 2, then cos (pi + n pi/2) will be 1, so its I think - cos (n pi/2) will be 1. I guess it works.
I dont quite get the minus before the sin (n pi/2), as pi is 0, then n pi/2 will be the value of n pi/2 so this one seems wrong.
 
Last edited:
Sonal7 said:
Yes i noticed that. cos pi is always -1, but i am not sure about the value of n as its a variable. So if n were 2, then cos (pi + n pi/2) will be 1, so its I think - cos (n pi/2) will be 1. I guess it works.
I dont quite get the minus before the sin (n pi/2), as pi is 0, then n pi/2 will be the value of n pi/2 so this one seems wrong.
Click to expand...
Look at the graph of cos(x). Pick any value of x (let's call it A). Note the value of cos(A). Now look at cos(pi + A).
 
You can also use the cos(a + b) ruleto figure out what cos(pi + x) =-cos(x). Or you could have used reference angles.
 
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