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GCSE Maths problem. Help!

The curve y=ax^2 + bx + c has a maximum point at (-4,7) and passes through (-5,5). Find the values of a,b and c
(edited 2 years ago)
Well you know that the curve passes through the points (-4,7) and (-5,5) so you can sub in these x,y values to get 2 equations in terms of a,b and c. You also know that dy/dx will equal zero when x=-4 because it is a maximum point - this should give you an equation in terms of just a and b. Using these 3 equations you should be able to work out a,b and c
(edited 2 years ago)
Reply 2
Avatar for mqb2766
mqb2766
21
An equivalent but slightly easier way which doesn't use derivatives is to just write it in complete the square form, so
a(x+4)^2 + 7
then use the extra point (-5,5) to get "a". Then expand to the usual quadratic form.
Reply 3
Avatar for Daisiee
Daisiee
OP
4
Original post by tej3141
Well you know that the curve passes through the points (-4,7) and (-5,5) so you can sub in these x,y values to get 2 equations in terms of a,b and c. You also know that dy/dx will equal zero when x=-4 because it is a maximum point - this should give you an equation in terms of just a and b. Using these 3 equations you should be able to work out a,b and c

great thank you
Reply 4
Avatar for Daisiee
Daisiee
OP
4
Original post by mqb2766
An equivalent but slightly easier way which doesn't use derivatives is to just write it in complete the square form, so
a(x+4)^2 + 7
then use the extra point (-5,5) to get "a". Then expand to the usual quadratic form.

Thanks
Got it!

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