-13=r/9+8
What's r stand for? I need help!

r is usually the radius but can also be a variable, in this case r = -189

Step-by-step explanation:

Solve for r:

-13 = r/9 + 8

Put each term in r/9 + 8 over the common denominator 9: r/9 + 8 = r/9 + 72/9:

-13 = r/9 + 72/9

r/9 + 72/9 = (r + 72)/9:

-13 = (r + 72)/9

-13 = (r + 72)/9 is equivalent to (r + 72)/9 = -13:

(r + 72)/9 = -13

Multiply both sides of (r + 72)/9 = -13 by 9:

(9 (r + 72))/9 = -13×9

(9 (r + 72))/9 = 9/9×(r + 72) = r + 72:

r + 72 = -13×9

9 (-13) = -117:

r + 72 = -117

Subtract 72 from both sides:

r + (72 - 72) = -72 - 117

72 - 72 = 0:

r = -72 - 117

-72 - 117 = -189:

r = -189

the cross section of an object is the shape you get when you cut straight through an object.

thus the cross section of the human hair is a circle. hence the cross sectional area is the area of a circle and area of circle is :

a = 3.14 × r²

we are given that the average width of human hair is = 4 × (1 / 10³) cm.

so the diameter of the circle is = 4 × (1 / 10³) cm.

∴ radius r = (diameter/2) = 2 × (1/10³) cm

now area a = 3.14 × (2 × (1/10³))² cm²

scientific notation is a standard way of writing very large and very small numbers so that they’re easier to both compare and use in computations. to write in scientific notation, follow the form

n × 10 ᵃ

where n is a number between 1 and 10, but not 10 itself, and a is an integer (positive or negative number).

so area can be rewritten in scientific notation as :

⇒ 3.14 × 4 × (1/10⁶)

⇒ 12.56 × 10 ⁻⁶ cm²

⇒1.256 × 10⁻⁷ cm²