Olympiad Maths Prep

Track / Stage 3 / 16 of 260 #16 of 2000

Problem 16

AMC 10/12, early questions
Geometry Difficulty 3.0 Find the answer

The angle of inclination of the line {x=2+tcos(30°)y=3tsin(60°)\begin{cases} x=-2+t\cos(\text{30°}) \\ y=3-t\sin(\text{60°}) \end{cases} (where tt is a parameter) is (  ).

A: 30°
B: 60°
C: 90°
D: 135°

Official solution

Given the parametric equations of the line:
\begin{cases}
x=-2+t\cos(\text{30°}) \\
y=3-t\sin(\text{60°})
\end{cases}

To find the angle of inclination of the line, we need to determine its slope (mm). From the parametric equations, we obtain the change in xx and yy with respect to the parameter tt:

\begin{align*}
\Delta x &= \cos(\text{30°}) \\
\Delta y &= -\sin(\text{60°})
\end{align*}

Therefore, the slope mm of the line is the ratio of Δy\Delta y to Δx\Delta x:

\begin{align*}
m &= \frac{\Delta y}{\Delta x} \\
m &= \frac{-\sin(\text{60°})}{\cos(\text{30°})} \\
m &= \frac{-\sqrt{3}/2}{\sqrt{3}/2} \\
m &= -1
\end{align*}

The slope of the line is equal to the tangent of its angle of inclination θ\theta with the positive direction of the x-axis, thus:
m=1=tan(θ) m = -1 = \tan(\theta)

To find the angle θ\theta that corresponds to tan(θ)=1\tan(\theta) = -1, we can consider angles in the second quadrant where the tangent is negative, and we know that tan(135°)=1\tan(135°) = -1.

Therefore, we conclude that the angle of inclination of the line is 135°\boxed{\text{135°}}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.