Lim x->0: Why Can't I Replace (x-sinx)/(sinx)^3 with 1/(sinx)^2-1/(sinx)^2?

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In summary, the conversation discusses the validity of replacing (x-sinx)/(sinx)^3 with 1/(sinx)^2-1/(sinx)^2, the use of the limit rule and L'Hôpital's rule in solving the limit, and the significance of the limit as x approaches 0. It is determined that the original expression cannot be simplified to the second expression, the limit rule cannot be applied, L'Hôpital's rule can be used, and the limit represents the slope of the tangent line and plays a significant role in calculus.
  • #1
yairl
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when i have lim x->0 (x-sinx)/(sinx)^3 why can't I replace it with 1/(sinx)^2-1/(sinx)^2?
thanks
 
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  • #2
because ##\frac{x-\sin{x}}{(\sin{x})^3}\not=\frac{1}{\sin^{2}{x}}-\frac{1}{\sin^{2}{x}}##. I understand that you want to use the limit ##\lim_{x\rightarrow 0}\frac{\sin{x}}{x}=1##, but in this case you need a better approximation than ##\sin{x}\sim x## as ##x\rightarrow 0## ...
 
  • #3
You know the de L'Hopital rule or Mc Laurin expansion?
 

FAQ: Lim x->0: Why Can't I Replace (x-sinx)/(sinx)^3 with 1/(sinx)^2-1/(sinx)^2?

1. Why can't I simply replace (x-sinx)/(sinx)^3 with 1/(sinx)^2-1/(sinx)^2?

Replacing (x-sinx)/(sinx)^3 with 1/(sinx)^2-1/(sinx)^2 is not valid because they are not equivalent. The former expression simplifies to 1/sinx, while the latter simplifies to 0. This is because in the numerator of the first expression, the x and sinx terms cannot be combined, while in the second expression, the x and sinx terms cancel each other out.

2. Can I use the limit rule "limit of a quotient is the quotient of the limits" to solve this limit?

No, you cannot use this rule in this scenario. This rule only applies when both the numerator and denominator functions have finite limits as x approaches the given value. In this case, both the numerator and denominator have a limit of 0, which is undefined.

3. Is there a way to simplify the expression (x-sinx)/(sinx)^3 to make it easier to solve?

Yes, there is a way to simplify the expression. By factoring out a sinx term from the numerator, we can rewrite the expression as (1-cosx)/sinx^2. This can then be simplified further to 1/sinx, which is equivalent to the original expression.

4. Can I use L'Hôpital's rule to solve this limit?

Yes, L'Hôpital's rule can be used to solve this limit. By taking the derivative of both the numerator and denominator, we can rewrite the expression as (1-cosx)/2sinx*cosx. This can then be simplified to 1/2cosx, which has a limit of 1/2 as x approaches 0.

5. What is the significance of the limit of (x-sinx)/(sinx)^3 as x approaches 0?

The limit of (x-sinx)/(sinx)^3 as x approaches 0 is significant because it represents the slope of the tangent line to the curve y=sinx at the point (0,0). This is known as the derivative of sinx at x=0, and has a value of 1 at this point. This limit also plays a crucial role in the development of calculus and the study of functions and their behavior.

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