What is the first step to show that two lines are perpendicular, by looking at the equations of the lines, if they are written in standard form?
a.
find the slope of each line.
b.
graph each equation.
c.
find the y-intercept of each line.
d.
re-write each equation in slope-intercept form.

Statements reasons 1. ab ? cd; ad ? bc 1. given 2. ac ? ac 2. reflexive property 3. ? adc ? ? cba 3. ? 4. ? dac ? ? bca; ? acd ? ? cab 4. cpctc 5. ? dac and ? bca are alt. int. ? s; ? acd and ? cab are alt. int. ? s 5. definition of alternate interior angles 6. ab ? cd; ad ? bc 6. converse of the alternate interior angles theorem 7. abcd is a parallelogram 7. definition of parallelogram what is the missing reason in step 3?