#1

The straight line

A) Determine the value of the constant

should know it, but cant think of how o do it atm :[

**y=2x + c**is a tangent to the curve**=x^2 + 6x + 7**.A) Determine the value of the constant

**c**.should know it, but cant think of how o do it atm :[

#2

**c**, I think, is the y-axis intercept.

That doesn't help.

#3

Differentiate the curve.

#4

if u differentiate the equation of the curve, its y=2x+6

edit:so c=6?

edit:so c=6?

#5

well thats what Im thinking but it doesnt quite seem to fit in my mind, dont know why.

on second thoughts it does work, It just eludes me exactly why.

on second thoughts it does work, It just eludes me exactly why.

*Last edited by dnjoe at Jan 14, 2007,*

#6

once you've differentiated it, you find what x equals when y is 0 I think. I should know this too... I outta revise maths more

Then sub that into the original equation and thats it...i think

Then sub that into the original equation and thats it...i think

#7

NO MATH NO MATH NO MATH!

Sorry.. had to get that out of my system..

Sorry.. had to get that out of my system..

#8

No when you differntiate the curve you get the gradient function of the curve. Because it is 2x the gradient of the tanget is 2. Therefore 2=2x+6, 2x=-4, x=-2. Sub x=-2 into y=2x+c and solve it for c when y=0.

#9

OK, I suck at maths.

#10

once you've differentiated it, you find what x equals when y is 0 I think. I should know this too... I outta revise maths more

Then sub that into the original equation and thats it...i think

no that would give you the gradient when y=0......aswell as the fact that we're dealing with the y-intercept(x=0) as opposed to the x-intercept(y=0).

I get it to be that when the tangent touches the curve they both have th same gradient so dy/dx = 2.

so 2x + 6 = 2

x = -2

when x = -2 y = -1

y = 2x + c

-1 = -4 + c

c = 3

doesnt seem right though.

#11

This is highschool stuff right ? I won't stand a chance in college if it is.

#12

well its calculus, no idea when you guys start that in the US. Not particularly complicated stuff though.