Question

Don't understand how to do this problem, i tried solving it and got A for an answer and my teacher said it was wrong...help please

Answer 1

Hey @satellite73 or @perl do you know how to answer this question?

Answer 2

what do you have so far?

Answer 3

first take the derivative

Answer 4

f ' (x) = 2ax + b

Answer 5

Well i had that a=1 and b=1 so them added together would equal 2, but i did not find the derivative of f(x)....that might be whats wrong

Answer 6

y = x + 4
tangent to
y = ax^2 + bx
it's slope at x = 2, y = 6 is
y'(2) = 2ax + b = 4a + b
is tangent to the line x+4, which has slope 1
tangent to that means slope -1
so
y'(2) = 4a + b = -1

and we also have that (2,6) is on the parabola:
4a + 2b = 6, that is 2a + b = 3

so we have to solve
4a + b = -1
2a + b = 3

so
2a = -4
a = -2
2a + b = 3 => -4+b =3, so b = 7
a + b = 5

Answer 7

y = x + 4 is tangent to f(x) at the point (2,6)

the 'slope' of the tangent line is 1 .

therefore
f ' (2) = 1

Answer 8

looks like abster got it :)

Answer 9

he is fast

Answer 10

isn't that parallel instead of tangent?

Answer 11

ok....@abtster 5 is not an answer choice

Answer 12

the curve y = x + 4 is tangent to f(x)

Answer 13

yeh so f'(2) = -1

Answer 14

so what would a+b=?

Answer 15

5

Answer 16

5 is not an answer choice

Answer 17

The question is wrong

Answer 18

My teacher got an answer

Answer 19

let me try

Answer 20

ok

Answer 21

ok....

Answer 22

if parallel was meant
then we have
4a+2b = 6 or
2a +b =3
4a+b =1 (if x+4 is parallel)
then
2a = -2
a = -1
and
back subst
-2 +b = 3
b = 5
a + b = 4
is one of the anwers: D

Answer 23

f ' (2) = 1
f(2) = 6

4a + b = 1
4a + 2b = 6

Answer 24

I got b = 5 , and a = -1

Answer 25

a + b = 4

Answer 26

ok awesome, so you both got 4

Answer 27

x+4 is parallel of cource that is tangent
stupid me

Answer 28

yeah i plugged into f ' (x) incorrectly, but it works now

Answer 29

ok, i get it now. Thank you both for your help!!

Answer 30

just given you your deserved medal, perl

Answer 31

thanks abtster :)