Angle between two vectors vecu=a_1hati+b_1hatj+c_1hatk→u=a1ˆi+b1ˆj+c1ˆk or < a_1,b_1,c_1><a1,b1,c1>
and vecv=a_2hati+b_2hatj+c_2hatk→v=a2ˆi+b2ˆj+c2ˆk or < a_2,b_2,c_2><a2,b2,c2> is given by
costheta=((vecu*vecv))/((|vecu|*|vecv|))cosθ=(→u⋅→v)(∣∣→u∣∣⋅∣∣→v∣∣),
where vecu*vecv=a_1a_2+b_1b_2+c_1c_2→u⋅→v=a1a2+b1b2+c1c2
and |vecu|∣∣→u∣∣ or |vecv|∣∣→v∣∣ are magnitudes of vectors vecu→u or vecv→v and here they are
sqrt(a_1^2+b_1^2+c_1^2)√a21+b21+c21 and sqrt(a_2^2+b_2^2+c_2^2)√a22+b22+c22
Hence angle between < -8,2,8><−8,2,8> and < 2,-3,5><2,−3,5> is given by
costheta=((-8)xx2+2xx(-3)+8xx5)/(sqrt((-8)^2+2^2+8^2)xxsqrt((2)^2+(-3)^2+5^2))cosθ=(−8)×2+2×(−3)+8×5√(−8)2+22+82×√(2)2+(−3)2+52
= (-16-6+40)/(sqrt(64+4+64)xxsqrt(4+9+25))−16−6+40√64+4+64×√4+9+25
= 18/(sqrt132xxsqrt38)18√132×√38
= 18/(11.4891xx6.1644)1811.4891×6.1644
= 0.254150.25415
and theta=75.28^oθ=75.28o