What is the Derivative of the Infinite Series of Tangent Powers?

In summary, the conversation is about finding the derivative of the infinite series s(x)=1-tan^2(x)+tan^4(x)-tan^6(x)+... where 0<x<pi/4, with the options A.sin2x, B.cos2x, C.-tan2x, D.-sin2x, and E.-cos2x. The attempt at a solution involved trying to derive the series but the person was unsure if they were missing something obvious. A hint was given to sum the geometric series for s(x) first, but the person was still having trouble. The content provider then asked about the series expansion of 1/(1+r) or the sum of a geometric series with first term a
  • #1
cj5892
2
0

Homework Statement


Consider the infinite series s(x)=1-tan^2(x)+tan^4(x)-tan^6(x)+... , where 0<x<pi/4
s'(x)=
A.sin2x
B.cos2x
C.-tan2x
D.-sin2x
E.-cos2x


The Attempt at a Solution



Attempting to derive the series, i get 1-2tanxsec^2x+4tan^3(x)sec^2(x)-6tan^5(x)sec^2(x)+...

am i missing something obvious here? i don't see any trig identities in that that would give any of the answers
 
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  • #2
cj5892 said:

Homework Statement


Consider the infinite series s(x)=1-tan^2(x)+tan^4(x)-tan^6(x)+... , where 0<x<pi/4
s'(x)=
A.sin2x
B.cos2x
C.-tan2x
D.-sin2x
E.-cos2x

The Attempt at a Solution



Attempting to derive the series, i get 1-2tanxsec^2x+4tan^3(x)sec^2(x)-6tan^5(x)sec^2(x)+...

am i missing something obvious here? i don't see any trig identities in that that would give any of the answers

Hint: Sum that geometric series for s(x) first.
 
Last edited:
  • #3
i still can't get it :(
 
  • #4
cj5892 said:
i still can't get it :(

What's the series expansion of 1/(1+r)?
 
  • #5
Or what is the sum of a geometric series with first term a and ratio r?
 

What is the definition of a derivative of an infinite series?

The derivative of an infinite series is the mathematical concept that represents the rate of change of the series at a particular point. It is calculated by taking the limit of the difference quotient of the series as the change in the independent variable approaches zero.

How is the derivative of an infinite series calculated?

The derivative of an infinite series can be calculated using various methods such as the power rule, product rule, quotient rule, or chain rule. These rules are applied to each term in the series and the resulting terms are summed together to get the derivative of the entire series.

What is the relationship between the derivative and the original infinite series?

The derivative of an infinite series represents the slope of the tangent line to the graph of the series at a certain point. This means that the derivative gives us information about the rate of change of the series and can help us analyze the behavior of the series at different points.

What are some real-world applications of the derivative of an infinite series?

The derivative of an infinite series has many practical applications in fields such as physics, engineering, economics, and finance. It is used to calculate rates of change, optimize functions, and model complex systems. For example, the derivative of a power series can be used to analyze the behavior of electrical circuits or the growth of populations.

Can the derivative of an infinite series always be calculated?

No, the derivative of an infinite series cannot always be calculated. In some cases, the series may not be differentiable at certain points or the derivative may not exist. Additionally, some series may be too complex to find an exact derivative and can only be approximated using numerical methods.

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