What is the value of the indicated (?) length ?

Which addition property is being illustrated below? (type commutative or associative) (6 + 7) + 1 = 6 + (7 + 1)

Given: and prove: what is the missing reason in the proof? given transitive property alternate interior angles theorem converse alternate interior angles theorem

Aplot of land has been surveyed for a new housing development with borders ab, bc, dc, and da. the plot of land is a right trapezoid with a height of 60 feet and an opposite leg length of 65 feet