Find the y -intercept of the oblique asymptote of the following curve.
y = x − 2 x 2 + 5 x + 6
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Put y = m x + c in the given equation and since the equation we need is not vertical therefore we can multiply both sides by ( x − 2 ) and get ( m x + c ) ( x − 2 ) = x 2 + 5 x + 6
⟹ ( m − 1 ) x 2 − ( 2 m − c + 5 ) x − ( 2 c + 6 ) = 0
This equation has double root at x − > ∞
So put coeff of x 2 = coefficient of x = 0
m − 1 = 2 m − c + 5 = 0
⟹ m − 1 = 0 , 2 m − c + 5 = 0
m = 1 , c = 7
So the y-intercept is c = 7
It's easier to use partial fractions to figure out what the graph looks like.
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Note that we can write the given equation as
y = x − 2 x 2 − 2 x + 7 x + 6 y = x − 2 x ( x − 2 ) + 7 x − 1 4 + 2 0 y = x + x − 2 7 ( x − 2 ) + 2 0 y = x + 7 + x − 2 2 0
Observe that when the absolute value of x is very very large, like for example x = 1 0 0 0 0 0 0 or x = − 3 0 0 0 0 0 0 , the fraction term on the right hand side becomes very small and y is approximately equal to x + 7 . When x → ± ∞ , y → x + 7 . Thus, y = x + 7 is an oblique asymptote of the given curve.
The y intercept of y = x + 7 is the value of y when x = 0 , that is y = 7 □