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  • Perplex

    Trig equations & identities (Lesson 2/5)

    The Unit Circle

    1 / 24

    Discussion

    Imagine we have a circle centered about the origin ​O(0,0)​ with radius ​1, and we place a point ​P​ on its circumference such that the line ​(OP)​ forms an acute angle of ​30°​ with the ​x​-axis:

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    (a)

    Find the coordinates of ​P.

    Solution:

    The hypotenuse of the triangle is the radius of the circle, which is 1. The triangle is placed so that its hypotenuse makes a ​30∘​ angle with the ​x​-axis, starting from the origin.


    To find the coordinates where the triangle (hypotenuse) meets the circle, we use the definitions of sine and cosine for a right triangle:

    ​
    sinθ=hypotenuseopposite​cosθ=hypotenuseadjacent​
    ​

    These are the SOH and CAH formulas.

    Since the hypotenuse is 1, the coordinates of the point on the circle at a ​30∘​ angle from the ​x​-axis are:

    ​
    (cos(30∘),sin(30∘))
    ​


    Evaluate ​cos(30∘)​ and ​sin(30∘)​ using a calculator:

    ​
    cos(30∘)sin(30°)​≈0.866(which is actually 2√3​)=0.5​
    ​

    So, the intersection point is:

    ​
    (0.866,0.5)
    ​