Using the shell method, the volume is
Each cylindrical shell has radius (the horizontal distance from the axis of revolution to the curve
); has height
(the vertical distance between a point on the
-axis in
and the curve
).
Compute the integral.
As given,
Step 1: Pick any three-digit counting number
Lets pick the number 643
Step 2: Reverse the digits of the original number.
So, reverse becomes 346
Now, it says that the difference of these two numbers has a tens digit of 9.
Lets, check:
So, its true that the difference has a tens digit of 9.
Another example:
Lets take 449
So reverse is = 944
Difference =
Here also the tens digit is 9.
So, the condition applies to all 3 digit numbers.