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Hello.
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So here we let our sum a sub n be a series.
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We let f of n be equal to a subn.
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And we let f be the function obtained by just replacing n with x.
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We get f of x equals 1 over 1 plus 16 times x squared.
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So if we take the derivative, we get that the derivative of f with respect to x is going to be equal to negative 32x over the quantity, 1 plus 16.
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X squared squared, which is going to be less than zero for every x grade equal to 1.
00:36
So we have that f of x is a decreasing function by the first derivative test.
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Now we show that f of x is going to be continuous.
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Well, if we let let x, let's say x not be an element from 1 to infinity, we let x sub n approach x not.
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So that's going to imply that x squared approaches x sub not squared.
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So we get that 1 plus 16x of n squared...