Geometry · Second Year
Geometry
Second Year · Angle facts & triangles · Tap NEXT to begin
Section 1 of 2
Angle Facts
Know these
Angles on a straight line add to $180^{\circ}$; vertically opposite angles are equal; parallel-line angles (F, Z, C).
Worked example — angles on a line
Three angles on a straight line are $x$, $2x$ and $60^{\circ}$. Find $x$.
$x + 2x + 60 = 180 \Rightarrow 3x = 120$
$x = 40^{\circ}$
Section 2 of 2
Triangles
Triangle facts
The three angles add to $180^{\circ}$. Isosceles: two equal sides give equal base angles. Equilateral: all $60^{\circ}$.
Now you try
You try
A right-angled triangle has angles $90^{\circ}$, $x$ and $2x$. Find $x$.
Pen and paper out — try it before you reveal.
$90 + x + 2x = 180 \Rightarrow 3x = 90$
$x = 30^{\circ}$
$x = 30^{\circ}$
You try
An isosceles triangle has a top angle of $70^{\circ}$ and equal base angles $A$. Find $A$.
Pen and paper out — try it before you reveal.
$2A + 70 = 180 \Rightarrow 2A = 110$
$A = 55^{\circ}$
$A = 55^{\circ}$
That’s Geometry.
Angle facts and triangle rules — the start of Junior Cert geometry.