7.a does not exclude the special case of a∈α; B does not rule out the special case of a∈β; C is obviously just a special case; Only answer d is correct.

8.k 1 = -a/( 1-a)，k2 = ( 1-a)/(2a+3)

When two lines are not parallel or perpendicular to the number axis, there is k 1 * k2 =-1. So:

k 1 * k2 = -a/(2a+3) = - 1

It can be solved as follows: a = -3.

But when the two straight lines are parallel or perpendicular to the numerical values respectively, in this example, when 1-a = 0, the two straight lines are also perpendicular to each other.

Therefore, the correct answer to this question is D.

14. Obviously, when x = y =-1, z = 2x+y = -3 is the minimum value.

15. Use Pythagorean theorem: | ab | = √ [(4-1) 2+(-2-3) 2+(3+5) 2] = √ 98 = 7 √ 2.