Question : If x + y = √3 and x – y = √2, the value of 8xy(x2 + y2) is:
Option 1: 6
Option 2: √6
Option 3: 5
Option 4: √5
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Correct Answer: 5
Solution : According to the question, x + y = √3 Squaring both sides, we get, ⇒ x 2 + y 2 + 2xy = 3 ––––––––––––(i) Now, x – y = √2 Squaring both sides, we get, ⇒ x 2 + y 2 – 2xy = 2 ––––––––––––(ii) Adding equations (i) and (ii), we get, 2x 2 + 2y 2 = 5 ⇒ x 2 + y 2 = $\frac{5}{2}$ ––––––––––––––––(iii) Now putting the value from equation (iii) in equation (i), we get, $\frac{5}{2}$ + 2xy = 3 ⇒ 2xy = 3 – $\frac{5}{2}$ ⇒ 2xy = $\frac{1}{2}$ $\therefore$ xy = $\frac{1}{4}$ Now, the value of 8xy(x 2 + y 2 ) = $8 × \frac{1}{4} × \frac{5}{2}$ = $5$ Hence, the correct answer is 5.
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