2x+3y=60 solve for x...
x=(60-3y)/2
p=xy^3, using x from above:
p=y^3(60-3y)/2
p=(60y^3-3y^4)/2
dp/dy=(180y^2-12y^3)/2
dp/dy=90y^2-6y^2
dp/dy=0 when:
90y^2-6y^3=0
6y^2(15-y)=0, we know y>0 so
y=15, since x=(60-3y)/2
x=7.5
(We solved it this way because otherwise you get quartic equations and cubic derivatives, yuck :P)
The equation of line a is obtained by multiplying the equation of line b by 4. i.e. there are linearly dependent and hence there are infinitely many solutions to the set of equations.
Let the amount invested at 4% be = x
Let the amount invested at 3% be = y
Given is:
or 
.... (1)
As, total income for the two investments is $194, so equation is:
....(2)
Putting value of x from (1) in (2)
And x=5200-y
Hence, money invested at 4% is $3800 and money invested at 3% is $1400
Answer: the 2nd and the 5th one
Step-by-step explanation:
