Therefore
l = ak, m = bk, n = ck (1)
But
l2 + m 2 + n2 = 1
Therefore
k2 (a2 + b2 + c2) = 1
or
k =
2
2
2
1
a
b
c
±
+
+
Fig 11 1
© NCERT
not to be republished
THREE D IMENSIONAL G EOMETRY
465
Hence, from (1), the d c