Rollercoaster Q WTF? (2 Viewers)

[electrolysis]

Gradient of RO = 1.2, which passes through (0,0)

y = ax^2 + bx
dy/dx = 2ax + b

at x = 0, m = 1.2

therefore, 1.2 = 2(a)(0) + b
b = 1.2

Gradient of QP = -1.8, and P is (30, y)

at x = 30, m = -1.8

therefore, -1.8 = 2(a)(30) + 1.2
-3 = 60a
a = - 1/20

a = -1/20
b = 1.2

ii) Equation is now: y = (-1/20)x^2 + 1.2x

vertex X = -b/2a
= 12

sub that into equation, y = 7.2

thus, v(12, 7.2)

now the distance D = vertical height from vertex to x-axis + height from x axis to Y coordinate of P

We know that P(30, y), sub 30 into the equation and you get -9

therefore P(30, -9)

D = 7.2 + 9
= 16.2

OMFG ARE YOU SERIOUS?!?!?!
I got part i, then didn't do part ii cause I thought part i was wrong >.>

fml. just lost an easy 2 marks

 

[VK2FMCR]

I got b=1.2, a =-1/20

I got that as well. Saw the next part went lolwat and skipped it.

 

[KAIZOKUsama]

OMG! When i saw this i was like 'I have no idea what the **** this shit is' so i moved on to the next question. When i got back to it with half an hour to go, i was like...ok, let's crank dat bullshit machine! and basically guessed the working out.

I CAN'T BELIEVE I ACTUALLY DID IT RIGHT!

 

[sinophile2]

Very easy question. Initially thought it was a parametric in my 2u paper.

 

[kushna]

What was so difficult about it?

 

[TechnIx]

Damn ya Brad >_<. I left part ii) since I -b/2a didn't spring to mind straight away, I had other marks to pick up I think...

Lol I didnt even think it was -b/2a. That didnt come to mind at all xD

I differentiated to find the stationary point, found out what x was, and then y at the peak. I also found out the y value of P, by subbing in x = 30 into the equation. then...

.:. d = (y value of peak) - (y value of P)
=7.2 - (-9)
=16.2