Question about related rates (calculus)

[fallen_6666]

Could someone help me with this question? The base of an isosceles triangle is 6 m. The other two sides are both increasing at 4m/s. Find the rate of increase of the area of the triangle when these sides are 5 m.

[BigMoosie]

My answer is [math]15m^2s^{-1}[/math]:

 

Base = 6

 

Side = 4t

 

Height = [math]\sqrt{16t^2-9}[/math]

 

Area = [math]f(t) = 3 (16t^2-9)^{0.5}[/math]

 

[math]f'(t) = 48t ( 16t^2 - 9)^{-0.5}[/math]

 

f'(1.25) = 15

[fallen_6666]

Thank you so much.

[BigMoosie]

No problem