Angle Between Vectors Given that a. b = | a || b |cos a b then it must follow that cos = a. b | a || b | … and this allows us to find the angle between a & b.
ExIfm = 2i - j + k and n = i + k Find the size of the angle between m & n in radians. ******** m = 2i - j + k so |m| = (2 2 +(-1) )= 6 n = i + k so |n| = ( )= 2 m.n = (2 X 1) + (-1 X 0) + (1 X 1)= = 3 (This is non-calculator !!) cos = m. n | m || n | = 3. 6 X 2 = 3. 12 = 3. 4 X 3 = = cos -1 ( 3 / 2 ) = 30°= / 6
Ex P is (3,2,1), Q is (7,0,5) & R is (11,2,-5). Find the size of QPR. ^ NAB *********** P Q R PQ = q – p = ( ) - ( ) = ( ) |PQ| = (4 2 + (-2) ) = 36 = 6 PR = r – p = ( ) - ( ) = ( ) |PR| = ( (-6) 2 ) = 100 = 10
PR.PQ = (4 X 8) + (-2 X 0) + (4 X (-6))= – 24 = 8 cos = PR.PQ |PQ||PR| = 8 6 X 10 = 2 / 15 = cos -1 ( 2 / 15 ) = 82.3°