If (s = 3i – j) and (t = 2i + 3j), find ((t – 3s).(t + 3s)).
-
A.
-77 -
B.
-71 -
C.
-53 -
D.
-41
Correct Answer: Option A
Explanation
(s = 3i – j; t = 2i + 3j)
( t – 3s = (2i + 3j) – 3(3i – j) = 2i + 3j – 9i + 3j = -7i + 6j)
(t + 3s = (2i + 3j) + 3(3i – j) = 2i + 3j + 9i – 3j = 11i)
((t – 3s).(t + 3s) = (-7i + 6j).(11i) = -77)