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they arent hard, i just cant seem to make them work.

the first one is:

A = (5,3,2)

B = (-1,4,6)

C = (8,2,0)

im using this format (x,y,z)

aA+bB+ C = a vector parallel to the y-axis.

so, that means i need to find a vector with only z components right?

well, the answers out of the back of the book are a = -12/7 and b = -4/7.

these dont work out.

heres the second one

A = (a,1,4)

B = (3,b,-6)

C = (5,-2,c)

Im supposed to find a,b, and c so that all three vectors are perpendicular to each other.

that means i should use the dot product with A and B, A and C, and B and C and set the dot product equal to zero for all three. So I did this, and i got answers for a, b and c but they dont work out when i check them.