the altitude of a triangle is increasing at a rate of 1 centimeters/minute while the area of the triangle is increasing at a rate of 2.5 square centimeters/minute. at what rate is the base of the triangle changing when the altitude is 11 centimeters and the area is 98 square centimeters?



Answer :

The base is decreasing at 1.16 centimeters/minute, when rate is the base of the triangle changing.

Let us assume,

The altitude = h

The base = b

The Area =  A

Area = [tex]\frac{1}{2} b*h[/tex]

[tex]\frac{dh}{dt}[/tex] is the rate at which the altitude is increasing

[tex]\frac{dA}{dt}[/tex] is the rate at which the Area is increasing

[tex]\frac{db}{dt}[/tex] is the changing rate of the base

We are to determine [tex]\frac{db}{dt}[/tex] when h = 11cm and A = 98 square centimeters

First we will determine at that time,

A = [tex]\frac{1}{2} b*h[/tex]

b = [tex]\frac{2A}{h}[/tex]

b = [tex]\frac{2*98}{11}[/tex]

b = 196/11

b = 17.8cm

Next, we will differentiate the formula A = [tex]\frac{1}{2} b*h[/tex] with respect to t.

That is,

[tex]\frac{d}{dt} (A)[/tex] = [tex]\frac{d}{dt} (\frac{1}{2} b*h)[/tex]

[tex]\frac{dA}{dt}[/tex] = [tex]\frac{1}{2} \frac{db}{dt} *h+\frac{1}{2} \frac{dh}{dt} *b[/tex]

Now, we can find [tex]\frac{db}{dt}[/tex]

[tex]\frac{dA}{dt}[/tex] = 2.5 square centimeters/minute

[tex]\frac{dh}{dt}[/tex] = 1 centimeters/minute

b = 17.8cm

h = 11 centimeters

Hence,

2.5 = [tex]\frac{1}{2} \frac{db}{dt} (11)+\frac{1}{2} (1)(17.8)[/tex]

2.5 = 5.5 [tex]\frac{db}{dt}[/tex] + 8.9

2.5 - 8.9 = 5.5 [tex]\frac{db}{dt}[/tex]

-6.4 = 5.5 [tex]\frac{db}{dt}[/tex]

[tex]\frac{db}{dt}[/tex] = -6.4/5.5

[tex]\frac{db}{dt}[/tex] = -1.16 [tex]\frac{cm}{min}[/tex]

This means the base is decreasing at 1.16 [tex]\frac{cm}{min}[/tex].

Therefore,

The base is decreasing at 1.16 centimeters/minute, when rate is the base of the triangle changing.

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