Question
Let an ellipse with centre and latus rectum of length have its major axis along -axis. If its minor axis subtends an angle at the foci, then the square of the sum of the lengths of its minor and major axes is equal to ____________.
Answer: 2
Solution
This problem requires a thorough understanding of the fundamental properties of an ellipse, including its defining equations, geometric relationships, and the significance of its parameters. Our objective is to leverage the given information about the latus rectum and the angle subtended by the minor axis at the foci to determine the lengths of the major and minor axes, and then compute a specific expression involving these lengths.
Key Concepts and Formulas for Ellipses
For an ellipse with its center at and its major axis along the x-axis:
- Standard Equation: The equation is given by .
- Here, represents the length of the semi-major axis ().
- represents the length of the semi-minor axis ().
- Crucially, for the major axis along the x-axis, we have .
- Length of Major Axis:
- Length of Minor Axis:
- Length of Latus Rectum (): This is the length of a chord passing through a focus and perpendicular to the major axis. It is given by .
- Eccentricity (): A dimensionless parameter that describes how "stretched" or "circular" the ellipse is. It's related to and by the equation . This can be rearranged to , which is often more convenient in calculations involving .
- Foci: The foci are located at . The distance from the center to each focus is .
- Geometric Property: Angle Subtended by Minor Axis at a Focus:
- Let the center be , an endpoint of the minor axis be , and a focus be .
- The minor axis connects and .
- The triangle is isosceles with .
- The line segment is perpendicular to the minor axis .
- Thus, in the right-angled triangle :
- The length of the side is (semi-minor axis).
- The length of the side is (distance from center to focus).
- If the minor axis subtends an angle at , then .
- From this right triangle, we can write . This relationship is key for problems involving this geometric condition.
Step-by-Step Solution
Step 1: Formulate an Equation from the Latus Rectum Length
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Understanding the given information: We are given an ellipse with its center at and its major axis along the x-axis. This confirms that the standard form applies, with and . We are also given that the length of the latus rectum is .
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Applying the Latus Rectum formula: The formula for the length of the latus rectum () is . We substitute the given value : Rearranging this equation to establish a relationship between and : Why this step is taken: The latus rectum length is a direct geometric property that provides an algebraic link between the semi-major axis () and the semi-minor axis (). This is our first independent equation, crucial for solving for two unknowns.
Step 2: Formulate an Equation from the Angle Subtended by the Minor Axis at the Foci
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Interpreting the angle condition: The problem states that "its minor axis subtends an angle at the foci." Let's consider one focus, , and the endpoints of the minor axis, and . The angle .
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Using the geometric property: As explained in the key concepts, the line segment connecting the center to the focus bisects the angle . Therefore, in the right-angled triangle :
- The angle .
- The side (opposite to ) has length (semi-minor axis).
- The side (adjacent to ) has length (distance from center to focus, where is the eccentricity).
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Applying trigonometry: We use the tangent function in : We know that .
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Eliminating eccentricity (): Our goal is to find and , so we need to eliminate . We can square both sides of the equation and then use the fundamental relationship . Squaring both sides: Now, substitute : Cross-multiplying gives us our second relation between and : Why this step is taken: This geometric property is the second independent piece of information provided. It introduces eccentricity (), which needs to be eliminated using the fundamental relation to obtain a second equation solely in terms of and .
Step 3: Solve the System of Equations for and }
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Our system of equations:
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Solving by substitution: Notice that the right-hand side of both equations is . This means we can equate the left-hand sides: From Equation 1, we have . Substitute this into Equation 2: Rearrange and solve for : Since represents the length of the semi-major axis, it must be a positive value (). Therefore, . This implies:
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Solving for : Now substitute the value of back into Equation 1: Since represents a length, it must be positive (): So, the semi-major axis length is and the semi-minor axis length is . Why this step is taken: We now have two distinct equations relating and . Solving this system is essential to find the specific values of the semi-axes, which define the dimensions of the ellipse.
Step 4: Calculate the Final Required Value
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Understanding the question: The problem asks for "the square of the sum of the lengths of its minor and major axes."
- Length of major axis =
- Length of minor axis =
- Sum of lengths =
- Required value =
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Substituting the values of and : Length of major axis = Length of minor axis =
Sum of lengths =
Square of the sum of lengths =
Wait! Let's recheck the problem statement and the correct answer. The correct answer is given as 2. My calculation yields 9. Let me re-examine the equations.
Equation 1: Equation 2:
If and , then . . Since , . If , then . These values seem correct based on the derivation.
Let me check the question and the "Current Solution" provided. The current solution has exactly the same steps and derivation. Step 3: Solve the System of Equations for and
- ... , . Step 4: Calculate the Final Required Value .
There might be a mismatch between the provided "Correct Answer: 2" and the problem statement/solution. Based on the problem statement and the derived and values, the answer should be 9.
Let's re-read the problem carefully: "the square of the sum of the lengths of its minor and major axes is equal to ____________." This means . If and , then and . Sum is . Square of sum is .
Perhaps there is a subtle misinterpretation of the latus rectum or the angle. Latus Rectum . Given . So . This is correct. Minor axis subtends at the foci. . . Using : . This is also correct.
The derivation for is robust. The calculation is also robust.
It is highly probable that the "Correct Answer: 2" provided in the prompt is incorrect, or the question implies something else that is not standard. Assuming the question and standard formulas are correct, the answer is 9. However, as an expert teacher, I should point out this discrepancy.
For the purpose of *rewriting the