Question The price of a sofa is Sx. A man buys the sofa on hire purchase according to the following terms:
a downpayment of 25 % and the remaining to be paid in monthly instalments over 30 months at a
simple interest rate of 12 % per annum. Given that his monthly instalment is $52, find the value of x.
The disk method will only involve a single integral. I've attached a sketch of the bounded region (in red) and one such disk made by revolving it around the y-axis.
Such a disk has radius x = 1/y and height/thickness ∆y, so that the volume of one such disk is
π (radius) (height) = π (1/y)² ∆y = π/y² ∆y
and the volume of a stack of n such disks is
where is a point sampled from the interval [1, 5].
As we refine the solid by adding increasingly more, increasingly thinner disks, so that ∆y converges to 0, the sum converges to a definite integral that gives the exact volume V,
