Step by step
The question
Question: find angle F.
Triangles ABC and DEF have the marked parts equal.
1 Match the parts
Congruent means the same shape and the same size.
One fits exactly on top of the other.
The order of the letters shows which corners match: A with D, B with E, C with F.
DEF is turned over.
That does not matter.
AB and DE are both 7 cm: a pair of equal sides.
S for side.
BC and EF are both 9 cm: a pair of equal sides.
S for side.
AC and DF are both 5 cm: a pair of equal sides.
S for side.
2 Find the rule
All three pairs are sides.
Side, side, side: S S S.
Three sides fix the triangle.
Like three sticks: they join up in only one way.
So S S S shows the triangles are congruent.
3 Use the matching parts
Angle F matches angle C: F with C.
In congruent triangles, matching parts are equal.
So angle F = angle C.
So angle F = 50°.
Let's look back.
First: match the parts.
Then: find the rule.
Last: use the matching parts.