1.

If area of triangle is 35 sq units with vertices `(2, - 6), (5, 4) a n d (k , 4)`. Then k is(A) 12 (B) `-2` (C) ` 12 , 2` (D) `12 , 2`

Answer» Area of a triangle is given by,
`A = 1/2|[x_1,y_1,1],[x_2,y_2,1],[x_3,y_3,1]|`
For the given triangle,
`35 = 1/2|[2,-6,1],[5,4,1],[k,4,1]|`
Applying `R_2->.R_2 - R_3` on the determinant
`=>|[2,-6,1],[5-k,0,0],[k,4,1]| = 70`
`=>2(0-0)-((-6)(5-k)-0)+1(4(5-k) = +-70`
`=>30-6k+20-4k = +-70`
`=>-10k = 20 and -10k = -120`
`=>k = -2 and k = 12`


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