Expert: Steve Holleran - 5/28/2008

Question
I think i have worked out this equation but i dont understand where i have gone wrong please could you help

y=11x+3
y=4x^2

11x+3=4x^2

4x^2-11x+3=0

x=11+ or - squared root (-11)^2-4*4*3/2*4

x= 11 + or - squared root 121-48/8

so x=11+ squared root 73/8=2.44 (to 3 sf)
or
x=11-squared root 73/8=0.31 (to 3sf)

i have tryed plugging in the answers to work out both y's but it doesn't work and i don't understand where i have gone wrong please help me

Answer
Hi Daryl,

Your quadratic formula work is fine, but the trouble is being caused by a sign mistake early on:

                 y = 11x + 3

                 y = 4x^2


so                4x^2 = 11x + 3

and               4x^2 - 11x - 3 = 0  .  You had "+3".

This then factors as (4x + 1)(x -3) = 0 so x = -1/4 or 3.

Its usually small things like this that cause complications.
Steve