Question

If the points (k,2-2k), (1-k, 2k) and (-k-4, 6-2k) are collinear, possible values of k are ..............

• A.
• B. 1/2
• C. 1
• D. -1

Answer 1

Answer: (B), (D)

The correct options are
B

1/2

D

-1

Points A(k,2-2k), B (1-k, 2k) and C(-k-4, 6-2k) are collinear.
Then, slope of line AB = slope of line BC
$\frac{2k-(2-2k)}{1-k-k}=\frac{6-2k-2k}{-k-4-1+k}$
$\frac{4k-2}{1-2k}=\frac{6-4k}{-5}$
$120k+10=6-4k-12k+8k^2$
$8k^2+4k-4=0$
$2k^2+k-1=0$
$2k^2(k+1)-1(k+1)=0$
$(2k-1)(k+1)=0$
2k-1=0 and k+1=0
$k=\frac{1}{2}andk=-1$
we can also say, slope of AB = slope of AC $\frac{2k-(2-2k)}{1-k-k}=\frac{6-2k-(2-2k)}{-k-4-k}$
$\frac{4k-2}{1-2k}=\frac{4}{-2k-4}$
$-8k^2+4k-16k+8=4-8k$
$8k^2+4k-4=0$
$2k^2+k-1=0$
we got the same equation of previous case
So $k=\frac{1}{2}$ and k=-1 are the only possible values of k when given points are collinear.