= 51.7 sq. inches.

Set right-hand index of the C scale over 8.1 on the D scale. Move the
indicator till hair-line is over .7854 (the special long mark near 8) at
the right hand of the B scale. Read the answer under the hair-line on
the A scale. Another way of finding the area of a circle is to set 7854
on the B scale to one of the indices of the A scale, and read the area
from the B scale directly above the given diameter on the D scale.

4. The circumference of a circle. Set the index of the B scale to the
diameter and read the answer on the A scale opposite PI on the B scale

Formula: C = PI * D
         C = 2 * PI * R

Example 60: The diameter of a circle is 1.54 inches, what is its
circumference?

Set the left-hand index of the B scale to 1.54 on the A scale. Read the
circumference 4.85 inches above PI on the B scale.

EXAMPLES FOR PRACTICE

61: What is the area of a circle 32-1/2 inches in diameter?
Answer 830 sq. inches

62: What is the area of a circle 24 inches in diameter?
Answer 452 sq. inches

63: What is the circumference of a circle whose diameter is 95 feet?
Answer 298 ft.

64: What is the circumference of a circle whose diameter is 3.65 inches?
Answer 11.5 inches

5. Ratio and Proportion.

Example 65:   3 : 7 : : 4 : X
or
(3/7) = (4/x)
Find X

Set 3 on C scale over 7 on D scale. Read X on D scale under 4 on C
scale. In fact, any number on the C scale is to the number directly
under it on the D scale as 3 is to 7.


PRACTICAL PROBLEMS SOLVED BY SLIDE RULE


1. Discount.
A firm buys a typewriter with a list price of $150, subject to a
discount of 20% and 10%. How much does it pay?

A discount of 20% means 0.8 of the list price, and 10% more means
0.8 X 0.9 X 150 = 108.

To do this on the slide rule, put the index of the C scale opposite 8 on
the D scale and move the indicator to 9 on the C scale. Then move the
slider till the right-hand index of the C scale is under the hairline.
Now, move the indicator to 150 on the C scale and read the answer $108
on the D scale. Notice that in this, as in many practical problems,
there is no question about where the decimal point should go.


2. Sales Tax.

A man buys an article worth $12 and he must pay a sales tax of 1.5%. How
much does he pay? A tax of 1.5% means he must pay 1.015 * 12.00.

Set index of C scale at 1.015 on D scale. Move indicator to 12 on C
scale and read the answer $12.18 on the D scale.

A longer but more accurate way is to multiply 12 * .015 and add the
result to $12.


3. Unit Price.

A motorist buys 17 gallons of gas at 19.5 cents per gallon. How much
does he pay?

Set index of C scale at 17 on D scale and move indicator to 19.5 on C
scale and read the answer $3.32 on the D scale.


4. Gasoline Mileage.

An automobile goes 175 miles on 12 gallons of gas. What is the average
gasoline consumption?

Set indicator over 175 on D scale and move slider till 12 is under
hair-line. Read the answer 14.6 miles per gallon on the D scale under
the left-hand index of the C scale.


5. Average Speed.

A motorist makes a trip of 256 miles in 7.5 hours. What is his average
speed?

Set indicator over 256 on D scale. Move slider till 7.5 on the C scale
is under the hair-line. Read the answer 34.2 miles per hour under the
right-hand index of the C scale.


6. Decimal Parts of an Inch.

What is 5/16 of an inch expressed as decimal fraction?

Set 16 on C scale over 5 on D scale and read the result .3125 inches on
the D scale under the left-hand index of the C scale.


7. Physics.