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Question

Let $f$ be a continuous function defined on $[0,2]$ such that $f(x) \ge 0$ for all $x \in [0,2]$. If the area bounded by $y = f(x)$, $x = 0$, $y = 0$ and $x=b$ is $\sqrt{3+b^2}-\sqrt{3}$, where $b\in (0,2]$, then $f(1)$ is equal to ______ (round off to 1 place of decimal).

The problem asks us to find the value of the function $f(1)$ given the area under the curve $y=f(x)$ from $x=0$ to $x=b$. We are given that the area, let's call it $A(b)$, is defined as:

$A(b) = \int_0^b f(x) dx = \sqrt{3+b^2} - \sqrt{3}$

Here, $f(x)$ is a continuous, non-negative function on $[0, 2]$, and $b \in (0, 2]$.

Relating Area and the Function using Calculus

According to the Fundamental Theorem of Calculus Part 1, the derivative of the definite integral function $A(b) = \int_0^b f(x) dx$ with respect to $b$ gives us the function $f(b)$.

So, we need to find the derivative of $A(b)$ with respect to $b$. $f(b) = \frac{d}{db} A(b) = \frac{d}{db} (\sqrt{3+b^2} - \sqrt{3})$

Calculating the Derivative

We differentiate the expression term by term:

  • The derivative of the constant term $\sqrt{3}$ is 0.
  • For the term $\sqrt{3+b^2}$, we use the chain rule. Let $u = 3+b^2$, then $\frac{du}{db} = 2b$. The derivative is $\frac{d}{db}(\sqrt{u}) = \frac{1}{2\sqrt{u}} \frac{du}{db}$.
  • Substituting back: $\frac{d}{db}(\sqrt{3+b^2}) = \frac{1}{2\sqrt{3+b^2}} \cdot (2b) = \frac{b}{\sqrt{3+b^2}}$.

Combining these, the derivative $f(b)$ is:

$f(b) = \frac{b}{\sqrt{3+b^2}} - 0 = \frac{b}{\sqrt{3+b^2}}$

Finding the Value of f(1)

Now we substitute $b=1$ into the expression for $f(b)$ to find $f(1)$.

$f(1) = \frac{1}{\sqrt{3+1^2}} = \frac{1}{\sqrt{3+1}} = \frac{1}{\sqrt{4}} = \frac{1}{2}$

The value of $f(1)$ is $\frac{1}{2}$, which is equal to 0.5.

Rounding to 1 decimal place, $f(1) = 0.5$.

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Important Questions from Area Under Curve

  1. Let $I$ be the integral defined as follows: $$I = \int_{0}^{1} \int_{0}^{\sqrt{y}} dx dy + \int_{1}^{2} \int_{\sqrt{y-1}}^{1} dx dy$$ If the order of the integration is changed, then which one of the following is the correct expression for $I$?
  2. Let $\alpha = \iint_S \vec{F} \cdot \hat{n} \, dS$, where $\vec{F} = (2x + 3z)\hat{i} + (xz - y)\hat{j} + (y^2 + 2z)\hat{k}$ and $S$ is the sphere with centre at $(3, -1, 2)$ and radius 9. Here, $\hat{n}$ is the unit normal drawn outward and $\hat{i}, \hat{j}, \hat{k}$ are unit vectors. 

    Then the value of $\frac{1}{36\pi} \alpha$ is equal to ________. (answer in integer)

  3. The value of $\frac{4}{\pi} \int_0^{\pi/2} \sin^2 x \text{ dx}$ is _________________ (rounded off to two decimal places).

  4. The work done by the force $F = (x + y)\hat{i} - (x^2 + y^2)\hat{j}$, where $\hat{i}$ and $\hat{j}$ are unit vectors in $\vec{OX}$ and $\vec{OY}$ directions, respectively, along the upper half of the circle $x^2 + y^2 = 1$ from $(1,0)$ to $(-1,0)$ in the $xy$-plane is

  5. If the line $y = \alpha x$, $\alpha \geq \sqrt{2}$, divides the area of the region 
    $R: = \{(x, y) \in \mathbb{R}^2| 0 \leq x \leq \sqrt{y}, 0 \leq y \leq 2\}$ 
    into two equal parts, then the value of $\alpha$ is equal to

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