Question Details

A line through (1,1,1) and perpendicular to both  i^ + 2j^ + 2k^ , 2i^ + 2j^ + k^  let (a,b,c) be foot of perpendicular from origin then 34 ( a+b+c ) is

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Correct Answer :

100

Solution :

The correct answer is 100.

Step-by-step Explanation:

Let the given vectors perpendicular to the line be:
v1 = i^ + 2 j^ + 2 k^
and
v2 = 2 i^ + 2 j^ + k^

The direction vector d of the line is perpendicular to both v1 and v2. Therefore, we can find d by taking the cross product of v1 and v2:
d = v1 × v2 = | i^ j^ k^ 1 2 2 2 2 1 <|

Expanding the determinant:
d = i^ ( 2 · 1 - 2 · 2 ) - j^ ( 1 · 1 - 2 · 2 ) + k^ ( 1 · 2 - 2 · 2 )
d = i^ ( 2 - 4 ) - j^ ( 1 - 4 ) + k^ ( 2 - 4 )
d = - 2 i^ + 3 j^ - 2 k^

Since the line passes through the point (1,1,1), any general point (a,b,c) on this line can be expressed in terms of a real parameter t as:
a = 1 - 2 t
b = 1 + 3 t
c = 1 - 2 t

We are given that (a,b,c) is the foot of the perpendicular from the origin (0,0,0) to the line. Therefore, the vector from the origin to this point, ai^+bj^+ck^, must be perpendicular to the direction vector of the line:
( a i^ + b j^ + c k^ ) · ( - 2 i^ + 3 j^ - 2 k^ ) = 0
- 2 a + 3 b - 2 c = 0

Substitute the expressions for a, b, and c in terms of t into this equation:
- 2 ( 1 - 2 t ) + 3 ( 1 + 3 t ) - 2 ( 1 - 2 t ) = 0
- 2 + 4 t + 3 + 9 t - 2 + 4 t = 0
17 t - 1 = 0
t = 1 17

Now, calculate the sum a+b+c:
a + b + c = ( 1 - 2 t ) + ( 1 + 3 t ) + ( 1 - 2 t )
a + b + c = 3 - t

Substituting t=117:
a + b + c = 3 - 1 17 = 51 - 1 17 = 50 17

Finally, find the value of 34(a+b+c):
34 ( a + b + c ) = 34 · 50 17 = 2 · 50 = 100

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