Q.If 7, x, y, 189 are in continued proportion, then the values of x and y respectively will be: (a) 63,21 (b) 21,23 (c) 21,63 (d) 23,21
Answer: C
If \(7, x, y, 189\) are in continued proportion, then \[ \frac{7}{x} = \frac{x}{y} = \frac{y}{189} \] From \(\frac{7}{x} = \frac{x}{y}\), we get: \[ y = \frac{x^2}{7} \quad \text{——— (i)} \] Again, from \(\frac{x}{y} = \frac{y}{189}\), we get: \[ y^2 = 189x \] Substituting the value of \(y\) from equation (i): \[ \left(\frac{x^2}{7}\right)^2 = 189x \] \[ \frac{x^4}{49} = 189x \] \[ x^3 = 189 \times 49 \] \[ x^3 = 3 \times 3 \times 3 \times 7 \times 7 \times 7 \] \[ x = 3 \times 7 = 21 \] Now, substituting \(x = 21\) into equation (i): \[ y = \frac{21 \times 21}{7} = 63 \]
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