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(a) The median of an equilateral triangle is increasing at the rate of 2√3 cm/s. Find the rate at which its side is increasing. OR (b) Sum of two numbers is 5. If the sum of the cubes of these numbers is least, then find the sum of the squares of these numbers.
UNDERSTAND COMPETENCY 5 Marks
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Calculation / Logic
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Target Level
MEDIUM
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UNDERSTAND COMPETENCY MEDIUM

Q: (a) The median of an equilateral triangle is increasing at the rate of 2√3 cm/s. Find the rate at which its side is increasing. OR (b) Sum of two numbers is 5. If the sum of the cubes of these numbers is least, then find the sum of the squares of these numbers.

Question Analysis & Solution

Detailed Solution

Step 1: Analyze option (a)

Let $a$ be the side of the equilateral triangle. The median of an equilateral triangle is given by $m = \frac{\sqrt{3}}{2}a$.

Step 2: Differentiate the median with respect to time

We are given that $\frac{dm}{dt} = 2\sqrt{3}$ cm/s. We need to find $\frac{da}{dt}$. Differentiating $m = \frac{\sqrt{3}}{2}a$ with respect to time $t$, we get: $$ \frac{dm}{dt} = \frac{\sqrt{3}}{2} \frac{da}{dt} $$

Step 3: Solve for the rate of change of the side

Substituting the given value of $\frac{dm}{dt}$, we have: $$ 2\sqrt{3} = \frac{\sqrt{3}}{2} \frac{da}{dt} $$ $$ \frac{da}{dt} = \frac{2 \cdot 2\sqrt{3}}{\sqrt{3}} = 4 $$ Thus, the rate at which the side is increasing is 4 cm/s.

Step 4: Analyze option (b)

Let the two numbers be $x$ and $y$. We are given that $x + y = 5$. We want to minimize $x^3 + y^3$.

Step 5: Express one variable in terms of the other

Since $x + y = 5$, we can write $y = 5 - x$. Then, we want to minimize $f(x) = x^3 + (5-x)^3$.

Step 6: Find the derivative of the function

$$ f(x) = x^3 + (5-x)^3 = x^3 + (125 - 75x + 15x^2 - x^3) = 15x^2 - 75x + 125 $$ $$ f'(x) = 30x - 75 $$

Step 7: Find the critical points

To find the critical points, set $f'(x) = 0$: $$ 30x - 75 = 0 $$ $$ x = \frac{75}{30} = \frac{5}{2} $$

Step 8: Verify that it is a minimum

$$ f''(x) = 30 > 0 $$ Since the second derivative is positive, $x = \frac{5}{2}$ corresponds to a minimum.

Step 9: Find the value of y

If $x = \frac{5}{2}$, then $y = 5 - x = 5 - \frac{5}{2} = \frac{5}{2}$.

Step 10: Calculate the sum of the squares

The sum of the squares is $x^2 + y^2 = (\frac{5}{2})^2 + (\frac{5}{2})^2 = \frac{25}{4} + \frac{25}{4} = \frac{50}{4} = \frac{25}{2}$.

Final Answer: (a) 4 cm/s OR (b) 25/2

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