Question

Find the vertex and length of the latus rectum for the parabola. y=-1/2(x-2)^2+8

Answer 1

Y = -(1/2)(x-2)² +8
Re write it in standard form:
(y-8) = -1/2(x-2)²  ↔ (y-k) = a(x-h)²

This parabola open downward (a = -1/2 <0), with a maximum shown in vertex
The vertex is (h , k) → Vertex(2 , 8)
focus(h, k +c )
 a = 1/4c → -1/2 = 1/4c → c = -1/2, hence focus(2, 8-1/2) →focus(2,15/2)

Latus rectum: y-value = 15/2

Replace in the equation y with 15/2→→15/2 = -1/2(x-2)² + 8
Or -1/2(x-2)² +8 -15/2 = 0
Solving this quadratic equation gives x' = 3 and x" = 2, then 
Latus rectum = 5