... với góc giữa hai VTPT 12 nn, . ) 121 2121 2222222 12 1 1122 2nnAABBCCnnABCABC.cos(),()..++==++++ hu • )00090(),()≤≤. • 121 212 0AABBCC()()⊥⇔++= Bài 1. Tính góc giữa ... phẳng: a) ))) 121 21022ambmcm;;,;;,;;==+=− b) 2 1121 122212ammbmmcmm(;;);(;;),(;;)=+−=++=+ c) ))) 121 2122 ammmbmmmc;;,;;,;;=+−=−+= d) )))13 2121 022abmmmcm;;,;;,;;=−=+−−=− Bài 14. Cho các ... a) 123 03 7125 0ABC(;;),(;;),(;;)− b) 01321 1123 171019ABC(;;),(;;),(;;)− c) 34753 2123 ABC(;;),(;;),(;;)−−−− d) 423211387ABC(;;),(;;),(;;)−− e) 3121 21113ABC(;;),(;;),(;;)−−−− 414074 312 ABC(;;),(;;),(;;)−−...