Michigan State University - Diagnostic Exam

5.8

  •   Start at the origin in the plane. Move one-third of the way toward the point (9, 12) and then turn 90 ∘ to the left and move 10 units. What are the coordinates of your final position?

    a) (- 7, 4)  b) (11, - 2)  c) (- 5, 2)  d) (3, 11 )  e) (- 5, 10 )

Solution:
So you are at the point (3, 4)  (see sketch) when you turn    ∘
90 counterclockwise. The equation of the line, L1   , through this point is given by y - 4 = - 3∕4 (x - 3)  . So we need to find all points that are 10 units from (3, 4)  and intersect L1   . That is, we must solve the system

                        - 3
             y - 4  =    4 (x - 3)
      2          2
(x - 3 ) + (y - 4)   =  100

It follows that

           (    )2
       2     - 3          2
(x - 3)  +    4    (x - 3)   =  100  ⇒
                25
                ---(x - 3)2  =  100  ⇒
                16
                          2     16-(100)
                   (x - 3)   =     25    ⇒
                         x   =  3 ± 8

It follows that the coordinates of the final position are (- 5, 10)  .

PIC

Perhaps is might be easier if we notice that the slope of L1   is -34 and that a triangle with base equal to 8 and height equal to 6 has a hypotenuse of length 10. So from (3, 4)  we move 8 units left and 6 units up (the blue path).

Diagnostic:

If you miss this question please review the Pythagorean Theorem.


July 15, 2008