Find The Sum Of Angles X and Y

Geometry Level 1

Find X + Y . X^\circ + Y^\circ.

6 0 60^\circ 9 0 90^\circ 12 0 120^\circ 15 0 150^\circ

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5 solutions

By theorem.
P T = P S PT=PS


Length of tangents from external point are equal. \color{#D61F06}{\text{Length of tangents from external point are equal.}}


This makes P T S \triangle PTS isosceles .
Using Angle Sum Property of \triangle .
y + y + 30 ° = 180 ° \Rightarrow y+y+30°=180°
y = 75 ° \Rightarrow \boxed{y=75°}
Now, by Alternate segment theorem
x = y = 75 ° \Rightarrow x=y=75°


Angle between tangent and chord at the point of contact is equal to the angle in the alternate segment. \color{#3D99F6}{\text{Angle between tangent and chord at the point of contact is equal to the angle in the alternate segment.}}


x + y = 75 ° + 75 ° = 150 ° \Rightarrow x+y=75°+75°=\boxed{150°}

First step should not be much problem. Joining center of circle to the tangents, isosceles triangle of 15 ^\circ , 150 ^\circ and 15 ^\circ appears. x = 15 0 2 \frac{150 ^\circ}{2} = 75 . ^\circ.

Lu Chee Ket - 5 years, 5 months ago

How 150 is wrong

Prakash Kumae - 2 years, 1 month ago
Nihar Mahajan
Dec 27, 2015

Note that Δ P T S \Delta PTS is isosceles and thus we have y + y + 3 0 = 18 0 y = 7 5 y+y+30^\circ=180^\circ \Rightarrow y=75^\circ . By Alternate segment theorem we have x = y = 7 5 x + y = 7 5 + 7 5 = 15 0 x=y=75^\circ \rightarrow x+y=75^\circ+75^\circ=\boxed{150^\circ}

Angel Krastev
Oct 5, 2016

In isosceles triangle who has angle 30, 2Y=150. In the circle X=Y because they measure with half of the same ark. So X+Y=2Y=150.

June Richardson
Jan 28, 2020

You don't even need to calculate the values of x and y. Notice that via the alternate segment theorem, the angle x is the same as the missing angle in the long triangle to the right. So the angles of the triangle are x, y, and 30. So the sum of x and y must be 180-30 =||150||

Joel Renjith
May 29, 2017

When two tangents are drawn from the same point i.e 30, they are the same length making it an isosceles triangle.

So y=(180-30)/2 =75

According to alternate segment theorem, x=y

So x=75 Therefore x+y=75 +75 =150 degrees

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