B. If 2x² + 6xy + 9y2 + 4x +12y + 3 = 0 represents a pair of straight lines, then 2 =
a)-3
b)-4
c) 1
d) 5
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Answer:
ANSWER
x
2
+6xy+9y
2
+4x+12y−5=0
Comparing the equation with the general equation of second degree gives
a=1,b=9,h=3,g=2,f=6,c=−5
Angle between a pair of straight lines that is tanθ=
∣
∣
∣
∣
∣
∣
a+b
2
h
2
−ab
∣
∣
∣
∣
∣
∣
tanθ=
∣
∣
∣
∣
∣
1+9
2
9−1×9
∣
∣
∣
∣
∣
=
9
0
tanθ=0
⇒θ=tan
−
(0)=0
∘
Angle between the pair of straight lines is zero therefore the lines are parallel.
Hence proved
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