Originally Posted by

**link0099** im sorry for posting so many questions. this is the last.

"the line $\displaystyle y=5x-3 $ is a tangent to the parabola $\displaystyle y=ax^2 + bx -1 $ at the point (1,2). find a and b"

now i have no idea where to start here. i could assume solving simultaneuosly could give some sort of answer but it would contain both unknowns.

if i differentiate $\displaystyle y=ax^2 + bx -1 $ i get

$\displaystyle y'= 2ax+b$

and from the tangent we know that

$\displaystyle 2ax + b = 5$ at (1,2)

its here that im confused. whether the whole previous section is wrong i just cant get an answer.