logx2y + logy2x = 1 and y = x2 - 30, then the value of x2 + y2 is:
Explanation:
Given, logx2y + logy2x = 1
⇒ logxy2 + logyx2 = 1
⇒ logxy + logyx = 2
Now, logxy and logyx are reciprocal of each other and their sum is 2. Sum of a number and its reciprocal is always greater than or equal to 2. Equality is true when the number and reciprocal are both equal to 1.
∴ logxy = 1 ⇒ x = y
Now, y = x2 - 30 [y = x] ⇒ x2 - x - 30 = 0 ⇒ (x - 6)(x + 5) = 0 ⇒ x = 6 [x = -5 is rejected as log is not defined for negative numbers.] ∴ y = 6
∴ x2 + y2 = 62 + 62 = 72
Hence, 72.
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