Question

Jensen wants to hang a mirror in his boat and put a frame around it. The mirror and frame must have an area of 19.75 square feet. The mirror is 4 feet wide and 5 feet long. Which quadratic equation can be used to determine the thickness of the frame, x? (2 points)
2x2 + 6x − 19.25 = 0

3x2 + 5x − 19.25 = 0

4x2 + 18x + 0.25 = 0

4x2 + 18x + 20 = 0

Answer 1

The third choice, 4x²+18x+0.25=0, is correct.

The frame will have the same thickness on both sides of the mirror.  Therefore we add 5+x+x to get the total length, and we have 4+x+x to get the total width. 

The area is found by multiplying the total width and total length:

A=(5+x+x)(4+x+x)
A=(5+2x)(4+2x)

Multiplying the binomials, we have:
A=5*4+5*2x+2x*4+2x*2x
A=20+10x+8x+4x²
A=20+18x+4x²

We know that the area is 19.75:
19.75=20+18x+4x²

We need quadratics set equal to 0, so subtract 19.75 from each side:
19.75-19.75 = 20+18x+4x²-19.75
0=0.25+18x+4x²


In standard form, it will be:
4x²+18x+0.25=0