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find equations of both lines through the point (2, −3) that are tangent to the parabola y = x2 + x

Sagot :

The equations of the tangents that pass through (2,−3) are y=−x−1 and  y= 11x−25.

What is an equation?

In arithmetic,an equation may be a formula that expresses the equality of 2 expressions, by connecting them with the sign =.

Main BODY:

The gradient of the tangent to a curve at any particular point is given by the derivative of the curve at that point. So for our curve (the parabola) we have

y=x²+x

Differentiating wrt x we get:

dy / dx = 2x +1

Let P(α,β) be any generic point on the curve. Then the gradient of the tangent at P is given by:

m=2α+1                   (using the derivative)

And as P lies on the curve we also have:

β=α²+α                    (using the curve equation)

And so the tangent at P passes through (α,α²+α) and has gradient 2α+1, so using the point/slope form y−y₁=m(x−x₁) the equation of the tangent at P is;

y−(α²+α)=(2α+1)(x−α)

if this tangent also passes through

(2,−3) then;

⇒-3−(α²+α)=(2α+1)(2−α)

⇒-3 −α²- α = 3α−2α²+2

⇒α²−4α−5=0

∴α=−1,5

If  α=−1⇒β=0

, and the tangent equation becomes:

y−0=(−1)(x+1)

∴ y=−x−1

If α=5⇒β=30

, and the tangent equation becomes:

y−30=(11)(x−5

∴y−30=11x−55

∴y=11x−25

Hence the equations of the tangents that pass through (2,−3) are y=−x−1 and  y= 11x−25.

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