m = 12 n = 8 | m+n | = 4 akar 7 Tentukan < (m,n)

|m+n|²=|m|²+|n|²+2|m||n| cos a
(4√7)²=12²+8²+2*12*8*cos
112=144+64+192cos a
192 cos a=112-144-64
192 cos a=-96
cos a=-96/192
cos a=-1/2
a=150°