This is a calculus question. The volume of a cube increases at a constant rate of 10 cm^3 per second. find the rate of change in its total surface area at the instant when its when its sides are 20 cm long.

2 cm^2/sec

Step-by-step explanation:

side of cube x cm

volume = x^3

d(volume)/dt = 3x^2 dx/dt

when c = 20, d(volume)/dt = 10 cm^3/sec

10 = 3(20^2) dx/dt

dx/dt = 10/1200 = 1/120 cm/sec

Surface area = SA = 6x^2

d(SA)/dt = 12x dx/dt

so when x = 20, as above

d(SA)/dt = 12(20)(1/120) cm^2/sec

= 2 cm^2/sec

fabric (square meters)                   cloaks

7                                                           21

4/3                                                         4

step-by-step explanation:

we are told that harry needs 2 square meters of fabric for every 6 wizard cloaks he makes.

1. first of all we will find how many cloaks can be made from 1 square meter of fabric by dividing 6 by 2.

we can see that 3 cloaks can be made from 1 square meter of fabric. now we will multiply 3 by 7 to find number of cloaks made from 7 square meters of fabric.

therefore, harry will make 21 cloaks from 7 square meters of fabric.

2. let us find how many square meters of fabric will be needed to make 4 cloaks.

first of all we will find fabric needed for 1 cloak by dividing 2 by 6.

we can see that fabric needed for 1 cloak is   square meter. now we will multiply   by 4 to find fabric needed for 4 cloaks.

it is 32.5

i hope i could u.