Question

The value of \( (\sin 70^\circ)(\cot 10^\circ \cot 70^\circ - 1) \) is:

• A. \( 1 \)
• B. \( 0 \)
• C. \( 3/2 \)
• D. \( 2/3 \)

Hint:

To simplify trigonometric expressions, remember identities such as \( \sin(90^\circ - x) = \cos x \) and use them to reduce the complexity of the expression.

Answer 1

The Correct Option is A

To evaluate the expression \( (\sin 70^\circ)(\cot 10^\circ \cot 70^\circ - 1) \), we proceed with the following steps:

1. Simplifying the Cotangent Product:
We use the trigonometric identity for the product of cotangents:

\[ \cot A \cot B - 1 = \frac{\cos A \cos B}{\sin A \sin B} - 1 = \frac{\cos A \cos B - \sin A \sin B}{\sin A \sin B} = \frac{\cos(A+B)}{\sin A \sin B} \]
Applying this to our expression with \( A = 10^\circ \) and \( B = 70^\circ \):

\[ \cot 10^\circ \cot 70^\circ - 1 = \frac{\cos(10^\circ + 70^\circ)}{\sin 10^\circ \sin 70^\circ} = \frac{\cos 80^\circ}{\sin 10^\circ \sin 70^\circ} \]

2. Substituting Back into the Original Expression:
Now, multiply by \( \sin 70^\circ \):

\[ (\sin 70^\circ)\left( \frac{\cos 80^\circ}{\sin 10^\circ \sin 70^\circ} \right) = \frac{\cos 80^\circ}{\sin 10^\circ} \]

3. Using Complementary Angle Identity:
We know that \( \cos 80^\circ = \sin(90^\circ - 80^\circ) = \sin 10^\circ \):

\[ \frac{\cos 80^\circ}{\sin 10^\circ} = \frac{\sin 10^\circ}{\sin 10^\circ} = 1 \]

Final Answer:
The value of the expression is \(\boxed{1}\).