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Question

If A and B are acute angles such that $2A + 2B = \pi$, then what is the maximum value of $\sin A \cdot \sin B$ ?

The correct answer is

1/2

Conditions and Objective

Given:

  • A and B are acute angles, meaning $0 < A < \frac{\pi}{2}$ and $0 < B < \frac{\pi}{2}$.
  • The constraint is $2A + 2B = \pi$.

Simplifying the constraint gives $2(A+B) = \pi$, which implies $A+B = \frac{\pi}{2}$. Angles A and B are complementary.

The objective is to find the maximum value of the product $P = \sin A \cdot \sin B$.

Derivation Using Identities

Since $A+B = \frac{\pi}{2}$, we have $B = \frac{\pi}{2} - A$. Substitute this into the expression for P:

  • $\sin B = \sin\left(\frac{\pi}{2} - A\right) = \cos A$.
  • Therefore, $P = \sin A \cdot \cos A$.

Using the trigonometric double angle identity $\sin(2A) = 2 \sin A \cos A$, we can write:

  • $P = \frac{1}{2} (2 \sin A \cos A) = \frac{1}{2} \sin(2A)$.

Maximum Value Calculation

We need to find the maximum value of $P = \frac{1}{2} \sin(2A)$ considering the range of A:

  • Since A is an acute angle ($0 < A < \frac{\pi}{2}$), the angle $2A$ lies in the interval $(0, \pi)$.
  • The sine function, $\sin(x)$, achieves its maximum value of 1 within the interval $(0, \pi)$.
  • The maximum occurs when $2A = \frac{\pi}{2}$, which gives $A = \frac{\pi}{4}$.
  • If $A = \frac{\pi}{4}$, then $B = \frac{\pi}{2} - \frac{\pi}{4} = \frac{\pi}{4}$. Both angles are indeed acute.

The maximum value based on this derivation is:

  • $P_{max} = \frac{1}{2} \times (\text{maximum value of } \sin(2A)) = \frac{1}{2} \times 1 = \frac{1}{2}$.

Final Answer Selection

The mathematically derived maximum value is $1/2$. However, selecting from the given options, the designated correct answer is Option B.

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Important Questions from Maxima and Minima

  1. Four small squares of side x are cut out of a square of side 12 cm to make a tray by folding the edges. What is the value of x so that the tray has the maximum volume?

  2. If 3 ≤ x ≤ 10 and 5 ≤ y ≤ 15 , then maximum value of \(\left(\frac{x}{y}\right)\) is-

  3. If N is a four digit number formed by digits x 1, x 2, x 3and x 4, then maximum value of \(\frac{N}{x_{1}+x_{2}+x_{3}+x_{4}}\) is-

  4. A wire of length 20 cm is to be bent into a rectangle. Which of the following statements is/are correct?

    I. The rectangle of the largest area is the square.
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    Select the answer using the code given below.

  5. Consider the following statements :

    Statement-I : 
    The function $f(x) = \frac{x^3 + 128}{x}$ has a minimum value 48 at $x = 4$.

    Statement-II : 
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    Which one of the following is correct in respect of the above statements?

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