If a(x1, y1), b(x2, y2), c(x3, y3) and d(x4, y4) form two lines segments, ab and cd, which condition needs to be met to prove ab _|_ cd?

Find the value of the cosine for angle a

Determine whether the quadrilateral below is a parallelogram. justify/explain your answer (this means back it up! give specific information that supports your decision. writing just "yes" or "no" will result in no credit.)

Koji is installing a rectangular window in an office building. the window is 823 feet wide and 534 feet high. the formula for the area of a rectangle is a=bh. what is the area of the window? enter your answer as a mixed number in simplest form in the box. $$ ft2