Difficulty: Medium
Correct Answer: x + y = -1
Explanation:
Introduction / Context:
This coordinate geometry question tests how to form a line equation from a point and an angle of inclination. The angle that a line makes with the positive X-axis determines its slope through the relation m = tan θ. Once the slope is known, the point-slope form y - y1 = m(x - x1) gives the line equation directly. The main care points are handling the negative angle (which creates a negative slope) and simplifying to a clean standard form.
Given Data / Assumptions:
Concept / Approach:
Use slope-angle relation:
m = tan θ.
Then use point-slope form:
y - y1 = m(x - x1).
Finally simplify into standard form (like x + y = constant) if possible. Since tan(-45°) = -1, the line has slope -1, which is a common and easy slope to simplify.
Step-by-Step Solution:
1) Compute slope from the given angle:
m = tan(-45°) = -1
2) Use point-slope form with (2, -3):
y - (-3) = -1(x - 2)
3) Simplify:
y + 3 = -x + 2
4) Bring x to the left and constants to the right:
x + y + 3 = 2
5) Simplify the constant:
x + y = -1
Verification / Alternative check:
Substitute the given point (2, -3) into x + y = -1:
2 + (-3) = -1, which matches the right side, so the line passes through the point. Also, the slope of x + y = -1 is -1 (since y = -x - 1), matching tan(-45°) = -1. Both conditions are satisfied.
Why Other Options Are Wrong:
• x + y = -5 and x - y forms correspond to different slopes and do not satisfy both the slope and point condition.
• y = x - 5 has slope +1, which would correspond to +45°, not -45°.
Common Pitfalls:
• Using tan(-45°) as +1 instead of -1.
• Misplacing the sign in y - y1 = m(x - x1), especially since y1 is negative here.
• Stopping at y + 3 = -x + 2 without simplifying to a clean final form.
Final Answer:
x + y = -1
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