Khan Academy
Khan Academy: Equation Practice With Angle Addition
Given the sum of a pair of angles and the algebraic expressions that represent them, form and solve an equation. Students receive immediate feedback and have the opportunity to try questions repeatedly, watch a video or receive hints.
AAA Math
Aaa Math: Finding the Third Angle of a Triangle
Finding the third angle of a triangle becomes easier by the explanation and examples presented.
Varsity Tutors
Varsity Tutors: Hotmath: Algebra Review: Angle Sum Theorem
Find a quick, concise explanation of the Angle Sum Theorem. A step by step explanation of the proof is included to help you understand the theorem.