Question

if two vectors 3i+4j+5k and 9i+12j+ck are parallel,the value of c?​

Answer 1

Answer:

given that 3i+4j+5k is parallel to 9i+12j+ck

3i+4j+5k X 3 =9i+12j+15k

from above c=15

Answer 2

Answer:

c=15

Explanation:

The vectors are parallel if their cross product is 0.

$\left[\begin{array}{ccc}i&j&k\\3&4&5\\9&12&c\end{array}\right] =0$

On solving the matrix,

⇒ i(4c-60) - j(3c-45) + k(36-36)=0

⇒i(4c-60)-j(3c-45)+0=0

⇒i(4c-60)+j(-3c+45)=0

⇒4c-60=3c-45

⇒ 4c-3c=60-45

⇒C=15

C=15.