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Evaluate Logarithmic Expression Involving Infinite Geometric Series

Let α=14+18+116+\alpha = \frac{1}{4} + \frac{1}{8} + \frac{1}{16} + \dots \infty and β=13+19+127+\beta = \frac{1}{3} + \frac{1}{9} + \frac{1}{27} + \dots \infty. Then the value of (0.2)log5(α)+(0.04)log5(β)(0.2)^{\log_{\sqrt{5}}(\alpha)} + (0.04)^{\log_5(\beta)} is equal to:

Options

A

4

B

5

C

8

Correct
D

25

Step-by-Step Solution

To evaluate the given expression, we first compute the values of the infinite geometric series α\alpha and β\beta.

Step 1: Calculate α\alpha The series for α\alpha is an infinite geometric progression with first term a1=14a_1 = \frac{1}{4} and common ratio r1=12r_1 = \frac{1}{2}: α=14+18+116+=14112=1412=12\alpha = \frac{1}{4} + \frac{1}{8} + \frac{1}{16} + \dots \infty = \frac{\frac{1}{4}}{1 - \frac{1}{2}} = \frac{\frac{1}{4}}{\frac{1}{2}} = \frac{1}{2}

Step 2: Calculate β\beta The series for β\beta is also an infinite geometric progression with first term a2=13a_2 = \frac{1}{3} and common ratio r2=13r_2 = \frac{1}{3}: β=13+19+127+=13113=1323=12\beta = \frac{1}{3} + \frac{1}{9} + \frac{1}{27} + \dots \infty = \frac{\frac{1}{3}}{1 - \frac{1}{3}} = \frac{\frac{1}{3}}{\frac{2}{3}} = \frac{1}{2}

Step 3: Evaluate the logarithmic expression We are required to find the value of: E=(0.2)log5(α)+(0.04)log5(β)E = (0.2)^{\log_{\sqrt{5}}(\alpha)} + (0.04)^{\log_5(\beta)}

Let's evaluate the first term: (0.2)log5(α)=(51)log51/2(12)(0.2)^{\log_{\sqrt{5}}(\alpha)} = (5^{-1})^{\log_{5^{1/2}}\left(\frac{1}{2}\right)} Using the property logbk(x)=1klogb(x)\log_{b^k}(x) = \frac{1}{k}\log_b(x): log51/2(12)=2log5(12)\log_{5^{1/2}}\left(\frac{1}{2}\right) = 2\log_5\left(\frac{1}{2}\right) Substituting this back into the first term: (0.2)log5(α)=5(2log5(12))=5log5((12)2)=5log5(4)=4(0.2)^{\log_{\sqrt{5}}(\alpha)} = 5^{-\left(2\log_5\left(\frac{1}{2}\right)\right)} = 5^{\log_5\left(\left(\frac{1}{2}\right)^{-2}\right)} = 5^{\log_5(4)} = 4

Now, let's evaluate the second term: (0.04)log5(β)=(52)log5(12)=52log5(12)=5log5((12)2)=5log5(4)=4(0.04)^{\log_5(\beta)} = (5^{-2})^{\log_5\left(\frac{1}{2}\right)} = 5^{-2\log_5\left(\frac{1}{2}\right)} = 5^{\log_5\left(\left(\frac{1}{2}\right)^{-2}\right)} = 5^{\log_5(4)} = 4

Step 4: Sum the terms E=4+4=8E = 4 + 4 = 8

Thus, the value of the given expression is 88, which corresponds to Option C.

Evaluate Logarithmic Expression Involving Infinite Geometric Series | Mathematics PYQ Solution - JEE Challenger