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Question

Let a,b,c be vectors of length 3,4 and 5 respectively. Let a be perpendicular to (b+c),b to (c+a) and c to (a+b). Find the length of the vector a+b+c.

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Solution

Given, |a|=3,|b|=4,|c|=5
Again a(b+c)
a.(b+c)=0 ....(1)
b(c+a)
b.(c+a)=0 ....(2)
c(a+b)
c.(a+b)=0 ....(3)
Adding (1), (2) and (3), we get
2(a.b+b.c+c.a)=0
Now |a+b+c|2=|a|2+|b|2+|c|2+2(a.b+b.c+c.a)
=32+42+52+0
=50
Thus the length of the vector |a+b+c|=52.

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