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Functions question

2025 · 24 Jan · Shift 1 · Q30
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  5. /2025 · 24 Jan · Shift 1 · Q30

Functions question

2025 · 24 Jan · Shift 1 · Q30

JEE MainMathematicsFunctionsMCQ+4 / −1
Let f(x)=2x+2+1622x+1+2x+4+32f(x)=\frac{2^{x+2}+16}{2^{2 x+1}+2^{x+4}+32}f(x)=22x+1+2x+4+322x+2+16​. Then the value of 8(f(115)+f(215)+…+f(5915))8\left(f\left(\frac{1}{15}\right)+f\left(\frac{2}{15}\right)+\ldots+f\left(\frac{59}{15}\right)\right)8(f(151​)+f(152​)+…+f(1559​)) is equal to
  1. A
    108
  2. B
    92
  3. C
    118
  4. D
    102
View written solutionFree

Correct answer: C

  1. Simplify the function

Given

f(x)=2x+2+1622x+1+2x+4+32. f(x)=\frac{2^{x+2}+16}{2^{2x+1}+2^{x+4}+32}.f(x)=22x+1+2x+4+322x+2+16​.

Let

Then

So

f(x)=4t+162t2+16t+32=4(t+4)2(t2+8t+16)=4(t+4)2(t+4)2=2t+4. f(x)=\frac{4t+16}{2t^2+16t+32} =\frac{4(t+4)}{2(t^2+8t+16)} =\frac{4(t+4)}{2(t+4)^2} =\frac{2}{t+4}.f(x)=2t2+16t+324t+16​=2(t2+8t+16)4(t+4)​=2(t+4)24(t+4)​=t+42​.

Hence

f(x)=22x+4. f(x)=\frac{2}{2^x+4}.f(x)=2x+42​.
  1. Find a useful symmetry relation

Now compute f(4−x)f(4-x)f(4−x):

f(4−x)=224−x+4. f(4-x)=\frac{2}{2^{4-x}+4}.f(4−x)=24−x+42​.

Since 24−x=162x,2^{4-x}=\frac{16}{2^x},24−x=2x16​, we get

f(4−x)=2162x+4=2⋅2x16+4⋅2x=2x2(2x+4). f(4-x)=\frac{2}{\frac{16}{2^x}+4} =\frac{2\cdot 2^x}{16+4\cdot 2^x} =\frac{2^x}{2(2^x+4)}.f(4−x)=2x16​+42​=16+4⋅2x2⋅2x​=2(2x+4)2x​.

Therefore

4f(x)=82x+4, 4f(x)=\frac{8}{2^x+4},4f(x)=2x+48​,

so

f(4−x)=1−4f(x)? f(4-x)=1-4f(x)?f(4−x)=1−4f(x)?

Let us verify directly:

4f(x)+f(4−x)=82x+4+2x2(2x+4)=16+2x2(2x+4)=1. 4f(x)+f(4-x)=\frac{8}{2^x+4}+\frac{2^x}{2(2^x+4)} =\frac{16+2^x}{2(2^x+4)} =1.4f(x)+f(4−x)=2x+48​+2(2x+4)2x​=2(2x+4)16+2x​=1.

Thus,

f(4−x)=1−4f(x). f(4-x)=1-4f(x).f(4−x)=1−4f(x).

Equivalently,

4f(x)+f(4−x)=1. 4f(x)+f(4-x)=1.4f(x)+f(4−x)=1.
  1. Set up the required sum

We need

8∑k=159f(k15).8\sum_{k=1}^{59} f\left(\frac{k}{15}\right).8k=1∑59​f(15k​).

Let

S=∑k=159f(k15).S=\sum_{k=1}^{59} f\left(\frac{k}{15}\right).S=k=1∑59​f(15k​).

Now note that

4−k15=60−k15.4-\frac{k}{15}=\frac{60-k}{15}.4−15k​=1560−k​.

So terms pair as

f(k15)withf(60−k15). f\left(\frac{k}{15}\right) \quad \text{with} \quad f\left(\frac{60-k}{15}\right).f(15k​)withf(1560−k​).

Using the identity,

4f(k15)+f(60−k15)=1.4f\left(\frac{k}{15}\right)+f\left(\frac{60-k}{15}\right)=1.4f(15k​)+f(1560−k​)=1.

Also,

4f(60−k15)+f(k15)=1.4f\left(\frac{60-k}{15}\right)+f\left(\frac{k}{15}\right)=1.4f(1560−k​)+f(15k​)=1.

Add these two equations:

5[f(k15)+f(60−k15)]=2.5\left[f\left(\frac{k}{15}\right)+f\left(\frac{60-k}{15}\right)\right]=2.5[f(15k​)+f(1560−k​)]=2.

Hence

f(k15)+f(60−k15)=25. f\left(\frac{k}{15}\right)+f\left(\frac{60-k}{15}\right)=\frac{2}{5}.f(15k​)+f(1560−k​)=52​.
  1. Count the pairs

The numbers k=1,2,…,59k=1,2,\dots,59k=1,2,…,59 form pairs

(1,59),(2,58),…,(29,31),(1,59),(2,58),\dots,(29,31),(1,59),(2,58),…,(29,31),

and the middle term is

k=30  ⟹  x=2.k=30 \implies x=2.k=30⟹x=2.

So there are 292929 pairs and one unpaired middle term.

Thus

S=29⋅25+f(2).S=29\cdot \frac{2}{5}+f(2).S=29⋅52​+f(2).

Now

f(2)=222+4=28=14. f(2)=\frac{2}{2^2+4}=\frac{2}{8}=\frac14.f(2)=22+42​=82​=41​.

Therefore

S=585+14=232+520=23720.S=\frac{58}{5}+\frac14 =\frac{232+5}{20} =\frac{237}{20}.S=558​+41​=20232+5​=20237​.
  1. Compute the required value
8S=8⋅23720=4745=94.8.8S=8\cdot \frac{237}{20}=\frac{474}{5}=94.8.8S=8⋅20237​=5474​=94.8.

This is not among the options, so let us instead use the identity more directly in a cleaner way.

From

4f(x)+f(4−x)=1,4f(x)+f(4-x)=1,4f(x)+f(4−x)=1,

for each pair (x,4−x)(x,4-x)(x,4−x),

4f(x)+f(4−x)=1,4f(x)+f(4-x)=1,4f(x)+f(4−x)=1, 4f(4−x)+f(x)=1.4f(4-x)+f(x)=1.4f(4−x)+f(x)=1.

Subtracting is not useful; adding gave

5(f(x)+f(4−x))=2,5(f(x)+f(4-x))=2,5(f(x)+f(4−x))=2,

which is correct. So the sum over each pair is indeed 25\frac2552​.

Then

S=29⋅25+14=23720,S=29\cdot\frac25+\frac14=\frac{237}{20},S=29⋅52​+41​=20237​,

and

8S=4745.8S=\frac{474}{5}.8S=5474​.

Since the answer must be an integer from the options, check whether the intended expression is actually

8(f(115)+f(215)+⋯+f(5915)).8\left(f\left(\frac{1}{15}\right)+f\left(\frac{2}{15}\right)+\cdots+f\left(\frac{59}{15}\right)\right).8(f(151​)+f(152​)+⋯+f(1559​)).

This still gives 94.894.894.8.

  1. Re-check the simplification carefully

Starting again,

f(x)=2x+2+1622x+1+2x+4+32. f(x)=\frac{2^{x+2}+16}{2^{2x+1}+2^{x+4}+32}.f(x)=22x+1+2x+4+322x+2+16​.

Let y=2xy=2^xy=2x. Then

f(x)=4y+162y2+16y+32=4(y+4)2(y+4)2=2y+4, f(x)=\frac{4y+16}{2y^2+16y+32} =\frac{4(y+4)}{2(y+4)^2} =\frac{2}{y+4},f(x)=2y2+16y+324y+16​=2(y+4)24(y+4)​=y+42​,

correct.

Then

f(4−x)=216/y+4=2y16+4y=y2(y+4). f(4-x)=\frac{2}{16/y+4}=\frac{2y}{16+4y}=\frac{y}{2(y+4)}.f(4−x)=16/y+42​=16+4y2y​=2(y+4)y​.

Hence

f(x)+f(4−x)=2y+4+y2(y+4)=4+y2(y+4)=12. f(x)+f(4-x)=\frac{2}{y+4}+\frac{y}{2(y+4)} =\frac{4+y}{2(y+4)}=\frac12.f(x)+f(4−x)=y+42​+2(y+4)y​=2(y+4)4+y​=21​.

This is the key correct relation.

So actually,

f(x)+f(4−x)=12. f(x)+f(4-x)=\frac12.f(x)+f(4−x)=21​.
  1. Now evaluate the sum correctly

Thus each pair satisfies

f(k15)+f(60−k15)=12. f\left(\frac{k}{15}\right)+f\left(\frac{60-k}{15}\right)=\frac12.f(15k​)+f(1560−k​)=21​.

There are 292929 such pairs, plus the middle term f(2)f(2)f(2).

So

S=29⋅12+f(2).S=29\cdot\frac12+f(2).S=29⋅21​+f(2).

And

f(2)=24+4=14. f(2)=\frac{2}{4+4}=\frac14.f(2)=4+42​=41​.

Thus

S=292+14=58+14=594.S=\frac{29}{2}+\frac14=\frac{58+1}{4}=\frac{59}{4}.S=229​+41​=458+1​=459​.

Therefore

8S=8⋅594=118.8S=8\cdot\frac{59}{4}=118.8S=8⋅459​=118.
  1. Compare with options

118118118 matches Option C.

So the correct answer is C: 118.

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