Find two positive numbers x and y such that their sum is 35 and the product x2 y5 is a maximum.
Let one number be x. Then, the other number is y = (35-x)
Let P(x)= x2 y5
Then, we get,
P(x) = 2x(35 –x)5 – 5x2(35-x)4
= x(35 –x)4 [2(35-x)– 5x]
= x(35 –x)4 [70-7x]
= 7x(35 –x)4(10-x)
Now, P’’(x) = 7(35 –x)4 (10-x)+7x[-(35 –x)4 – 4(35-x)3 (10-x)]
= 7(35 –x)4 (10-x) - 7x(35 –x)4 – 28x(35-x)3 (10-x)]
=7(35 –x)3 [(35-x)(10-x) - x(35 –x) – 4x(10-x)]
=7(35 –x)3 [350-45x+x2-35x+x2-40x+4x2]
=7(35 –x)3 [6x2-120x+350]
Now, P’(x) = 0
⇒ x = 0, 35, 10
When x = 0, 35 This will make the product x2y5 equal to 0.
Therefore, x= 0, 35 cannot be possible values of x.
And when x = 10
Then, we have,
P’’(x) =7(35 –10)3[6(10)2-120(10)+350]
= 7(25)3 [-250]<0
Then, by second derivative test,
x = 10 and y = 35 -10 = 25 is the point of local maxima of P.
Therefore, the required number are 10 and 25.