Ian Everhart
IB Calculus, pd. 8
6 November 2000
Extra Credit
1. Let f(x) = x8 -2x + 3 and x0 = 2.
Find limh->0 (f ' (x0 + h) - f ' (x0)) / h

f ' (x) = 8 x7 - 2 (using shortcut for polynomial functions) Thus:

limh->0 (f ' (x0 + h) - f ' (x0)) / h

= limh->0 ((8(2 + h)7 - 2 - (8(2)7 - 2)) / h

= limh->0 ((8(h7 + 14h6 + 84h5 + 280h4 + 540h3 + 672h2 + 448h = 128) - 2 - (8(128) - 2)) / h

= limh->0 ((8h7 + 112h6 + 672h5 + 2240h4 + 4480h3 + 5376h2 + 3584h + 1024 - 2) - (1024 - 2)) / h

= limh->0 (8h7 + 112h6 + 672h5 + 2240h4 + 4480h3 + 5376h2 + 3584h + 1024 - 2 - 1024 + 2)) / h

= limh->0 (8h7 + 112h6 + 672h5 + 2240h4 + 4480h3 + 5376h2 + 3584h) / h

= limh->0 (h(8h6 + 112h5 + 672h4 + 2240h3 + 4480h2 + 5376h + 3584)) / h

= 8(0)6 + 112(0)5 + 672(0)4 + 2240(0)3 + 4480(0)2 + 5376(0) + 3584

= 3584 = d2/dx2 [x8 -2x + 3] |x=2


2. Q(t) = 200(30-t)2
(a) dQ/dt | x=10 = ?
(b) The average rate of draining from t = 0 to t = 10 = ? (a) Q(t) = 200(30-t)2

= 200 (900 - 60t - t2)

= 180,000 - 12,000t + 200t2

= 200t2 - 12,000t + 180,000

Thus, Q ' (t) = 400t - 12,000

When t = 10, Q ' (t) = Q' (10) = 400(10) - 12,000

= 4,000 - 12,000

= -8,000

Therefore, at t = 10, ten minutes after the tank begins to drain, the tank is emptying at a rate of 8,000 gallons/minute.

(b) At t = 0, Q (t) = 200(30-0)2 = 200 * 900 = 180,000 At t = 10, Q (t) = 200(30-10)2 = 200 * 400 = 80,000

Average rate of drain = {delta}Q(t)/{delta}t = (Q(t)f - Q(t)0) / (tf - t0)

= (180,000 - 80,000) / (10 - 0) = 100,000 / 10

= 10,000 gallons/minute