Trig derivative applications answer check

[spoc21]

Homework Statement

The base of an isosceles triangle is 20 cm and the altitude is increasing at the rate of 1 cm/min. At what rate is the base angle increasing when the area is 100 cm^2?

Homework Equations

The Attempt at a Solution

Im very unsure about my solution method. I have attached a copy my solution in pdf format, and would really appreciate if someone could take a quick look at it, and preferably tell me if its right or wrong..Thanks

 

[Mark44]

Homework Statement

The base of an isosceles triangle is 20 cm and the altitude is increasing at the rate of 1 cm/min. At what rate is the base angle increasing when the area is 100 cm^2?

Homework Equations

The Attempt at a Solution

Im very unsure about my solution method. I have attached a copy my solution in pdf format, and would really appreciate if someone could take a quick look at it, and preferably tell me if its right or wrong..


Thanks

Looks good with one very minor exception tan^-1(1) = pi/4 (exactly), and is only approximately equal to .7854.