Problem Solving: If 3+2=7, 5+4=23, 7+6=47 and 9+8=79 to what is 10+9 equal? If theta is the angle between the diagonals of a parallelogram ABCD whose vertices are A(0,2) , B (2,-1) , C(4,0) and D(2,3). show that tan theta =2?