Answer:
AC = 25 units
Step-by-step explanation:
Given ∠ A = ∠ C then the triangle is isosceles and BC = AB , that is
7x - 1 = 5x + 5 ( subtract 5x from both sides )
2x - 1 = 5 ( add 1 to both sides )
2x = 6 ( divide both sides by 2 )
x = 3
Then
AC = 6x + 7 = 6(3) + 7 = 18 + 7 = 25 units
It's a value you should probably memorize:
You can derive it using some trigonometric identities, other known values of cosine, and properties of the cosine function. For example, using the double angle identity for cosine:
If 
, then
and you probably know that 
, so
When we take the square root, we should take the positive root because 
whenever 
:
The formula for perimeter is 2l+2w. Since we know l=(w-4)-9, we can substitute this into the formula.
l=w-13
2(w-13)+2w=143
2w-26+2w=143
4w=169
w=42.25 units
Hope this helps!
$680 was the original price!
100-62=38
258.40 x 100= 25840
25840/38=680
