If \(s = 3i – j\) and \(t = 2i + 3j\), find \((t – 3s).(t + 3s)\).
-
-77
- -71
- -53
- -41
The correct answer is: A
Explanation
\(s = 3i - j; t = 2i + 3j\)
\( t - 3s = (2i + 3j) - 3(3i - j) = 2i + 3j - 9i + 3j = -7i + 6j\)
\(t + 3s = (2i + 3j) + 3(3i - j) = 2i + 3j + 9i - 3j = 11i\)
\((t - 3s).(t + 3s) = (-7i + 6j).(11i) = -77\)