If P(0, 1, 2), Q(4, -2, 1) and O(0, 0, 0) are three points, then -POQ =
π6
π4
π3
π2
PQ2=(4−0)2+(−2−1)2+(1−2)2
= 16 + 9 + 1 = 26
OP2=(0−0)2+(1−0)2+(2−0)2
= 0 + 1 + 4 = 5
QO2=(0−4)2+(0+2)2+(0−1)2
= 16 + 1 + 4 = 21
Since, PQ2=OP2+QO2
Hence, ∠POQ=π2