NCERT Class XI Mathematics - Straight Lines - Solutions

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Question : 27
Total: 74
Find equation of the line passing through the point (2, 2) and cutting off intercepts on the axes whose sum is 9.
Solution:  
Let the intercepts made by the line on the x-axis and y-axis be a and 9 – a respectively. Then its equation is
x
a
+
y
9a
= 1
Since it passes through point (2, 2), we have
2
a
+
2
9a
= 1
⇒ 2(9 – a) + 2a = a(9 – a) ⇒ 18 – 2a + 2a = 9a – a2
⇒ 18 = 9a – a2a2 – 9a + 18 = 0 ⇒ a2 – 6a – 3a + 18 = 0
⇒ a(a – 6) – 3 (a – 6) = 0 ⇒ a = 3, 6
Now, if a = 3 ⇒ b = 9 – 3 = 6 and if a = 6 ⇒ b = 9 – 6 = 3
So, required equation is
x
3
+
y
6
= 1 or
x
6
+
y
3
= 1
i.e., 2x + y – 6 = 0 or x + 2y – 6 = 0.
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