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# 急 Functions 40分連答案

(38) The fig. shows the dimensions of an L-shaped cardboard. The perimeter of the cardboard is 30 cm. Let A cm^2 be the area of the cardboard

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(38a) Express y in terms of x

(38b) Express A as a function of x. State the domain of the function

(38c) Find the maximum area of the cardboard and the cooresponding value of x

(40) In the fig., triangle ABC is an isosceles triangle, where AB = 8 cm and AC = BC = 5 cm. A rectangle PQRS is inscribedin triangle ABC and SC = RC. Suppose SR = x cm

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(40a) Show that the area of PQRS is (3x - (3/8)x^2) cm^2

(40b) Find the area of the largest rectangle that can be inscribed in triangle ABC

Answers:

(38a) y = 11 - x

(38b) A = -x^2 + 12x + 3; the domain is the collectiom of the real number x, where 0 < x < 11

(38c) max. area = 39 cm^2 when x = 6

(40b) 6 cm^2

### 1 Answer

- ?Lv 58 years agoFavorite Answer
38a.

y + 3 + 1 + (x+3) + (y+1) + x = 30

2x + 2y + 8 = 30 => x + y = 11

y = 11 - x

38b.

A = x(y+1) + 3(1)

A = x(11-x+1) + 3

A = x(12-x) + 3

A = 12x - x^2 + 3 and 0 < x < 11

38c.

by completing the square

A = -(x^2 - 12x - 3)

A = -(x^2 - 12x + 36 -36 - 3)

A = -(x^2 - 12x + 36) + 39

A = -(x - 6)^2 + 39

as -(x - 6)^2 always < 0

Maximum of A = 39 when x = 6

40a.

height of C from AB = sqrt[5^2 - (8/2)^2] = 3

triangle ABC similar to triangle SRC

CS/AC = SR/AB = RC/BC = height of C from SR / height of C from AB

x/8 = height of C from SR / 3

height of C from SR = 3x/8

height of rectangular PQRS = 3 - (3x/8)

area of PQRS = x[3 - (3x/8)]

40b.

by completing the square

A = 3x - (3x^2)/8 = -(3/8)[x^2 - 8x]

A = -(3/8)[x^2 - 8x + 16 - 16]

A = -(3/8)[(x-4)^2 - 16]

A = -(3/8)(x-4)^2 + 6

as -(x - 4)^2 always < 0

Maximum of A = 6 when x = 4

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