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                                    Find the value of P if the line 3x + 4y – P = 0 is a tangent to the circle x2 + y2 = 16. | 
                            
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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 c2 = 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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