Students will see a derivation for the area of an obtuse triangle.

Do you Believe the Area Formula for Triangles? (Tanton Mathematics) by DrJamesTanton

Next, give your students these challenges:

- 2019 Math Kangaroo Levels 1-2 Problem #23 by STEM4all
- 2018 Math Kangaroo Levels 7-8 Problem #29 by STEM4all

Cycling Squares by NRICH: The trick is to find numbers which can be paired with only one or two of the remaining numbers. Such pairs, or triples, must occur in the loop. Notice, for example, that 2 can only be paired with 14, to make 16, or 34, to make 36. Thus, the sequence 14, 2, 36, or its reverse, must occur in the loop. Next, you would remove 2 from your list of available numbers, because it's surrounded, namely, by 14 and 34. Then you'd continue in this way until no numbers remain. Reading the loop clockwise, starting at 2, the solution is: 2, 34, 30, 19, 17, 8, 28, 21, 15, 10, 6, 3, 13, 12, 4, 5, 11, 14.

Conclude by leading this investigation:

Skinny Man Tango (area, algorithm)

by MathPickle