Posted by Anique on Monday, March 14, 2011 at 11:05am.
There are 3 vectors:
p= 7i + 10j
q= 3i + 12j
r= i +4j
Could you show me how to find the angle PQR. using the scalar product.

Maths  Reiny, Monday, March 14, 2011 at 1:58pm
I will associate your vectors to have endpoints
P(7,10), Q(3,12) and R(1,4)
then vector(PQ) = (4,2)
vector(QR) = (4,8)
then PQ.QR = PQQRcos(theta), where theta is the angle between them
1616 = PQQRcos(theta)
cos(theta) = 0
theta = 90 degrees
(notice the slope of lines PQ and QR are negative reciprocals of each other, so PQ and QR are perpendicular)
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