If y = mx + 4 is a tangent to both the parabolas, y² = 4x and x² = 2by, then b is equal to:
If y = mx + 4 is a tangent to both the parabolas, y² = 4x and x² = 2by, then b is equal to:
Option 1 -
-32
Option 2 -
-128
Option 3 -
-64
Option 4 -
128
-
1 Answer
-
Correct Option - 2
Detailed Solution:The tangent to the parabola y² = 4ax is y = mx + a/m.
For y² = 4x, a=1. So, the tangent is y = mx + 1/m.
The given line is y = mx + 4.
Comparing the two, 1/m = 4 ⇒ m = 1/4.
The line is y = (1/4)x + 4.
This line is also tangent to x² = 2by.
Substitute y into the parabola equation:
x² = 2b (1/4)x + 4)
x² = ( b/2 )x + 8b
x² - ( b/2 )x - 8b = 0.
For tangency, the discriminant (D) is zero.
D = (-b/2)² - 4 (1) (-8b) = 0.
b²/4 + 32b = 0.
b ( b/4 + 32) = 0.
b = 0 (not possible) or b/4 = -32 ⇒ b = -128.
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⇒
&
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