NIMCET 2016 Mathematics PYQ — The line touches the ellipse if k is… | Mathem Solvex | Mathem Solvex
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NIMCET 2016 — Mathematics PYQ
NIMCET | Mathematics | 2016
The line 3x+5y=k touches the ellipse 16x2+25y2=400 if k is
Choose the correct answer:
A.
±√5
B.
±√15
C.
±25
(Correct Answer)
D.
±35
Correct Answer:
±25
Explanation
Concept: The standard form of the equation of ellipse: a2x2+b2y2=1 where center coordinates are (0, 0), a = length of semi-major axis, and b = length of semi-minor axis For a line y = mx + c to be tangent of such ellipse where m is the slope of the line and c is a constant, then: c^2 = a^2m^2 + b^2 condition must follow, i.e., y = mx ± √(a^2m^2 + b^2) is the standard form of tangent to </span><br><spanstyle="font−size:14pt;">a2x2+b2y2=1 ellipse
Calculation: The given ellipse equation: 16x2+25y2=400 It can be rewritten in standard form as, 25x2+16y2=1 ⇒a=25=5 ⇒b=16=4 The equation of a line given: 3x+5y=k, ⇒y=5−3x+5k ∴m=5−3,c=5k For the line to be tangent, c2=a2m2+b2 ⇒(5k)2=52(5−3)2+42 ⇒25k2=9+16=25 ⇒k2=625 ⇒k=±25
Explanation
Concept: The standard form of the equation of ellipse: a2x2+b2y2=1 where center coordinates are (0, 0), a = length of semi-major axis, and b = length of semi-minor axis For a line y = mx + c to be tangent of such ellipse where m is the slope of the line and c is a constant, then: c^2 = a^2m^2 + b^2 condition must follow, i.e., y = mx ± √(a^2m^2 + b^2) is the standard form of tangent to </span><br><spanstyle="font−size:14pt;">a2x2+b2y2=1 ellipse
Calculation: The given ellipse equation: 16x2+25y2=400 It can be rewritten in standard form as, 25x2+16y2=1 ⇒a=25=5 ⇒b=16=4 The equation of a line given: 3x+5y=k, ⇒y=5−3x+5k ∴m=5−3,c=5k For the line to be tangent, c2=a2m2+b2 ⇒(5k)2=52(5−3)2+42 ⇒25k2=9+16=25 ⇒k2=625 ⇒k=±25