Q. 25: The points (2 , 1) and (-3 , -4) are opposite vertices of a parellelogram. If the other two vertices lie on the line x + 9y + c = 0, then c is
25. C.
The line x + 9y + c = 0, forms one of the diagonal of the parallelogram.
The other diagonal connects the points (2, 1) and (-3, -4).
The diagonals of a parellogram bisect each other, hence the line x + 9y + c = 0 should pass hrough the midpoint of the line joining (2, 1) and (-3, -4).
The midpoint of the line joining (2, 1) and (-3, -4) = = = (-0.5 , -1.5)
Therefore, the line x + 9y + c = 0 passes through (-0.5 , -1.5),
Hence, (-0.5) + 9(-1.5) + c = 0
(-0.5) + 27(-0.5) + c = 0
28 (-0.5) + c = 0
-14 + c = 0
c = 14.
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