1.

Find the value of P if the line 3x + 4y – P = 0 is a tangent to the circle x2 + y2 = 16.

Answer»

The condition for a line y = mx + c to be a tangent to the circle x2 + y2 = a2 is c2 = a2 (1 + m2

Equation of the line is 3x + 4y – P = 0 

4y = -3x + P 

y = \(\frac{-3}{4}x+\frac{P}{4}\)

∴ m = \(\frac{-3}{4}\), c = \(\frac{P}{4}\)

Equation of the circle is x2 + y2 = 16 

∴ a2 = 16

Condition for tangency we have c= a2(1 + m2)

⇒ \((\frac{P}{4})^2\) =  16(1 + \(\frac{9}{16}\))

⇒ \(\frac{P^2}{16}\) = 16(\(\frac{25}{16}\))

⇒ P2 = 16 x 25 

⇒ P = ±√16√25 

⇒ P = ±4 x 5 

⇒ P = ±20



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