The straight line 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 :[
Differentiate the curve.
if u differentiate the equation of the curve, its y=2x+6

edit:so c=6?
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.
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Last edited by dnjoe at Jan 14, 2007,
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 MATH NO MATH NO MATH!

Sorry.. had to get that out of my system..
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.
OK, I suck at maths.
Quote by shmooty
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.
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