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Mathematics
Problem
Which one of the following relations among the direction cosines
l
,
m
,
n
l,\,m,\,n
l
,
m
,
n
of any vector is always true? a)
l
2
+
m
2
−
n
2
=
1
l^{2}+m^{2}-n^{2}=1
l
2
+
m
2
−
n
2
=
1
b)
l
+
m
+
n
=
1
l+m+n=1
l
+
m
+
n
=
1
c)
l
2
+
m
2
+
n
2
=
1
l^{2}+m^{2}+n^{2}=1
l
2
+
m
2
+
n
2
=
1
d)
l
m
+
m
n
+
n
l
=
1
lm+mn+nl=1
l
m
+
mn
+
n
l
=
1
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