NCERT Class XI Mathematics - Straight Lines - Solutions

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Question : 56
Total: 74
Find the equations of the lines, which cut-off intercepts on the axes whose sum and product are 1 and – 6, respectively.
Solution:  
Suppose the intercepts be a and b
Given : Sum of intercepts = 1 i.e. a + b = 1 ... (i)
and product of intercepts = – 6 i.e. ab = – 6 ... (ii)
From (i) & (ii), we get
a(1 – a) = – 6 ⇒ a2 – a – 6 = 0 ⇒ a = 3, –2.
Case (I) : If a = 3, then b =
6
a
=
6
3
= - 2
∴ Equation of the line is
x
3
+
y
2
[Since x/a +y/b = 1]
⇒ – 2x + 3y = – 6 ⇒ 2x – 3y – 6 = 0
Case (II) : If a = – 2, then b =
6
a
=
6
2
= 3
Thus equation of the line is
x
2
+
y
3
= 1
⇒ 3x – 2y = – 6 ⇒ 3x – 2y + 6 = 0.
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