calculus (1 Viewer)

[gummibear0731]

hey,

ive been trying to solve this calculus question from challenge exercise 8, but i dont understand the final part of the question.

question: find the point of intersection between the tangents to the curve y=x^3-2x^2-5x+3 at the points where x =2 and x=-1.

And so i find the equations of both tangets and get: 2x=y+7=0 and x+y+5=0
These intersect at x=-4 and y=-1... ie. (-4,-1).

The answer however is (-4,-73). I realised that if I subbed x into the original equation, out popped the -73 for y, but i dont understand why you would do this, I only tried this after desperately trying to find the y value in the answers. test in a couple of days - help!

 

[InteGrand]

hey,

ive been trying to solve this calculus question from challenge exercise 8, but i dont understand the final part of the question.

question: find the point of intersection between the tangents to the curve y=x^3-2x^2-5x+3 at the points where x =2 and x=-1.

And so i find the equations of both tangets and get: 2x=y+7=0 and x+y+5=0
These intersect at x=-4 and y=-1... ie. (-4,-1).

The answer however is (-4,-73). I realised that if I subbed x into the original equation, out popped the -73 for y, but i dont understand why you would do this, I only tried this after desperately trying to find the y value in the answers. test in a couple of days - help!

We're not meant to sub. the x-value into the cubic, we are meant to do it in the line equations (which you did originally I think).

 

[gummibear0731]

We're not meant to sub. the x-value into the cubic, we are meant to do it in the line equations (which you did originally I think).

Yeah that's what i thought... do you think the textbook has the wrong answer?

 

[InteGrand]

Yeah that's what i thought... do you think the textbook has the wrong answer?

Is it MIF? I think that textbook often has incorrect answers.

 

[kashkow]

I'm getting same answers as you OP. I think the answers are just wrong...

m1 = 2 with point at (-1, 5)

y-5=2(x+1)
y-5=2x+2
2x-y+7=0


m2 = -1 (2, -7)

y+7=-1(x-2)
y+7=-x+2
x+y+5=0

x=-4

y=-1