Tell whether y=-3(x+1)^2+4 has a minimum or maximum value. then find that value.

Question

tamtoan · Answer

negative coefficient, what do you think? parabola up or down?

anonymous · Answer

differentiate this eqn and equate it to zero to find the x value 
substitute the x value to find whether your getting minimum or maximum value

anonymous · Answer

uhmm.isnt it a maximum ? i just dont know how to find the value.

tamtoan · Answer

maximum is correct, then use the method chandhuru to get the value :) if you haven't learn calculus, i think there are formula for finding the vertex isn't it?, i just don't remember what it is :)

anonymous · Answer

no im not in calculus lol. im taking it next year. i think the formula is -b/2(a) . i got 1/2 but im not sure if thats correct

tamtoan · Answer

the value should be a point, it has (x,y) coordinate :)

anonymous · Answer

so what i did was wrong  ? >_< .

anonymous · Answer

wait so , is the x value 1/2 ? but i just need to plug it in to get y?

tamtoan · Answer

what is b and what is 2a in this function?

tamtoan · Answer

or equation? if you set y = 0

anonymous · Answer

well form y=-3(x+1)^2+4  = -3x^2-3+4 so b is -3 & a is -3 right ?

anonymous · Answer

from*

tamtoan · Answer

no...expansion of (x+1)^2 = (x+1)(x+1) = x^2 + 2x + 1, try again :)

anonymous · Answer

what about the -3? where does that fit in ? >_<

tamtoan · Answer

after expand that, you distribute the -3 in...then simply...only until you have the equation in the form...ax^2 + bx + c = 0 ...then you can correctly know the value of a,b and c

tamtoan · Answer

simply = simplify

anonymous · Answer

wait ... so it will be -3(x+1)(x+1)+4 ?

tamtoan · Answer

no, what is (x +1)(x+1) equal to ?

anonymous · Answer

x^2+2x+1

tamtoan · Answer

ok then what is -3(x^2 + 2x +1) equal to ? distribute the -3 inside

anonymous · Answer

-3x^2-6x-3 ?

tamtoan · Answer

yes then you also have +4 ..rewrite the whole equation with +4

anonymous · Answer

so -3x^2-6x-1

tamtoan · Answer

-3 +4 = 1 :) it's not -1...also after that, your equation is -3x^2 - 6x +1 = 0...now what is your a and b and c ?

anonymous · Answer

oh yea lmfao . so a is -3 b is -6 & c is 1

tamtoan · Answer

:) then using formula to find x and y coordinate of the vertex

anonymous · Answer

so i plugged it into the eqtn . -b/2a & the answer is -1 ?

anonymous · Answer

well for x.

tamtoan · Answer

yeah, you got x, now sub in and find y...then you have the value (the point) of the max.

anonymous · Answer

okay thanks !!

tamtoan · Answer

np :) enjoy and have a good weekend