Post

Created by @nathanedwards
 at November 2nd 2023, 1:10:54 pm.

AP Calculus AB Exam Question:

Find the limit algebraically:

limxπ2cos(x1)sin2(2x2)\lim_{x \to \frac{\pi}{2}} \frac{\cos(x-1)}{\sin^2(2x-2)}

Step-by-Step Solution:

To find the limit as xx approaches π2\frac{\pi}{2}, we can use algebraic techniques such as factoring and simplifying.

Let's start by simplifying the expression:

limxπ2cos(x1)sin2(2x2)\lim_{x \to \frac{\pi}{2}} \frac{\cos(x-1)}{\sin^2(2x-2)}

Recall the identity cos(x1)=cos(x)cos(1)+sin(x)sin(1)cos(x-1) = cos(x)cos(1) + sin(x)sin(1). Using this, the expression becomes:

limxπ2cos(x)cos(1)+sin(x)sin(1)sin2(2x2)\lim_{x \to \frac{\pi}{2}} \frac{cos(x)cos(1) + sin(x)sin(1)}{\sin^2(2x-2)}

Next, observe the denominator sin2(2x2)\sin^2(2x-2). We know that sin(2θ)=2sin(θ)cos(θ)\sin(2\theta) = 2\sin(\theta)\cos(\theta). Applying this identity, we have:

limxπ2cos(x)cos(1)+sin(x)sin(1)(2sin(x1)cos(x1))2\lim_{x \to \frac{\pi}{2}} \frac{cos(x)cos(1) + sin(x)sin(1)}{(2\sin(x-1)\cos(x-1))^2}

Simplifying further, we get:

limxπ2cos(x)cos(1)+sin(x)sin(1)4sin2(x1)cos2(x1)\lim_{x \to \frac{\pi}{2}} \frac{\cos(x)cos(1) + \sin(x)\sin(1)}{4\sin^2(x-1)\cos^2(x-1)}

Now, we need to analyze the numerator and the denominator separately.

Let's evaluate the numerator first. For the limit as xx approaches π2\frac{\pi}{2}:

limxπ2cos(x)cos(1)+sin(x)sin(1)\lim_{x \to \frac{\pi}{2}} \cos(x)cos(1) + \sin(x)\sin(1)

Using the limit properties, we can evaluate each term separately. Recall that limxπ2cos(x)=0\lim_{x \to \frac{\pi}{2}} \cos(x) = 0 and limxπ2sin(x)=1\lim_{x \to \frac{\pi}{2}} \sin(x) = 1. Plugging these values into the expression, we get:

0cos(1)+1sin(1)=sin(1)0 \cdot \cos(1) + 1 \cdot \sin(1)= \sin(1)

Next, let's evaluate the denominator. For the limit as xx approaches π2\frac{\pi}{2}:

limxπ24sin2(x1)cos2(x1)\lim_{x \to \frac{\pi}{2}} 4\sin^2(x-1)\cos^2(x-1)

Using the limit properties, we can evaluate each term separately. Recall that limxπ2cos(x1)=0\lim_{x \to \frac{\pi}{2}} \cos(x-1) = 0 and limxπ2sin(x1)=1\lim_{x \to \frac{\pi}{2}} \sin(x-1) = 1. Plugging these values into the expression, we get:

4(1)2(0)2=04 \cdot (1)^2 \cdot (0)^2 = 0

Finally, we can put the evaluated numerator and denominator together to find the overall limit.

limxπ2cos(x)cos(1)+sin(x)sin(1)4sin2(x1)cos2(x1)=sin(1)0\lim_{x \to \frac{\pi}{2}} \frac{\cos(x)cos(1) + \sin(x)\sin(1)}{4\sin^2(x-1)\cos^2(x-1)} = \frac{\sin(1)}{0}

However, since the denominator evaluated to zero, we need to further analyze the behavior around π2\frac{\pi}{2}. For this, we can examine the behavior of the original numerator and denominator.

Looking at the original expression, we know that sin2(2x2)\sin^2(2x-2) approaches zero as xx approaches π2\frac{\pi}{2}, while cos(x1)\cos(x-1) does not. Therefore, the limit of the expression does not exist algebraically as it is undefined.

Hence, the final answer is:

limxπ2cos(x1)sin2(2x2) is undefined.\lim_{x \to \frac{\pi}{2}} \frac{\cos(x-1)}{\sin^2(2x-2)} \text{ is undefined.}