diketahui a=2i-5j dan b=-2i+5j, besar sudut antara kedua vector adalah

a = alfa

cos a = ((2,-5) . (-2,5))/(akar(2²+(-5)²) . akar((-2)²+(5)²)
cos a = -29/29
cos a = -1
a = 180°

a = 2i - 5j => |a| = √(2^2 + -5^2) = √(4 + 25) = √29
b = -2i + 5j => |b| = √(-2^2 + 5^2) = √(4 + 15) = √29
a . b = 2(-2) + (-5)(5) = -4 - 25 = -29

a . b = |a| |b| cos ¢
-29 = √29 . √29 cos ¢
-29 = 29 cos ¢
cos ¢ = -1
cos ¢ = cos 180°
¢ = 180°