Correct Answer - Option 1 :
\(\rm \frac{365}{4}\)
Given:
x + y = 5
\(\rm \frac{1}{x}+\frac{1}{y}=\frac{20}{9}\)
Formula used:
(x3 + y3) = (x + y)(x2 - xy + y2)
(x + y)2 = x2 + 2xy + y2
Calculation:
\(\rm \frac{1}{x}+\frac{1}{y}=\frac{20}{9}\)
⇒ (y + x)/xy = 20/9
⇒ 5/xy = 20/9
⇒ xy = 9/4 (1)
Also, (x + y)2 = x2 + 2xy + y2
⇒ 52 - 2xy = x2 + y2
⇒ 25 - 2 × 9/4 = x2 + y2 (From 1)
⇒ 25 - 9/2 = x2 + y2
⇒ (50 - 9)/2 = x2 + y2
⇒ x2 + y2 = 41/2 (2)
Now, (x3 + y3) = (x + y)(x2 - xy + y2)
⇒ 5 × (41/2 - 9/4)
⇒ 5 × 73/4
⇒ 365/4
∴ (x3 + y3) = 365/4