MCQMediumJEE 2026Trigonometric Ratios & Identities

JEE Mathematics 2026 Question with Solution

The sum of all the real solutions of the equation

log(x+3)(6x2+28x+30)=52log(6x+10)(x2+6x+9)\log_{(x+3)}(6x^2 + 28x + 30) = 5 - 2\log_{(6x+10)}(x^2 + 6x + 9) is equal to

  • A

    11

  • B

    00

  • C

    22

  • D

    44

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: log(x+3)(6x2+28x+30)=52log(6x+10)(x2+6x+9)\log_{(x+3)}(6x^2 + 28x + 30) = 5 - 2\log_{(6x+10)}(x^2 + 6x + 9)

Find: The sum of all real solutions.

Factor the polynomial terms:

6x2+28x+30=(x+3)(6x+10)6x^2+28x+30 = (x+3)(6x+10)

and

x2+6x+9=(x+3)2x^2+6x+9 = (x+3)^2

Substitute into the equation:

log(x+3)((x+3)(6x+10))=52log(6x+10)((x+3)2)\log_{(x+3)}((x+3)(6x+10)) = 5 - 2\log_{(6x+10)}((x+3)^2)

Using logarithmic properties:

1+log(x+3)(6x+10)=54log(6x+10)(x+3)1 + \log_{(x+3)}(6x+10) = 5 - 4\log_{(6x+10)}(x+3)

Let

y=log(x+3)(6x+10)y = \log_{(x+3)}(6x+10)

Then

log(6x+10)(x+3)=1y\log_{(6x+10)}(x+3) = \frac{1}{y}

using

logab=1logba\log_a b = \frac{1}{\log_b a}

So the equation becomes

1+y=54y1 + y = 5 - \frac{4}{y}

Multiplying by yy:

y2+y=5y4y^2 + y = 5y - 4 y24y+4=0y^2 - 4y + 4 = 0 (y2)2=0(y-2)^2 = 0

Hence,

y=2y=2

Therefore,

log(x+3)(6x+10)=2\log_{(x+3)}(6x+10) = 2

which gives

6x+10=(x+3)2=x2+6x+96x+10 = (x+3)^2 = x^2+6x+9

So,

x2=1x^2 = 1 x=1,1x = 1, -1

Check validity:

  • For x=1x=1, base is 44 and argument is 6464, so it is valid.
  • For x=1x=-1, base is 22 and argument is 88, so it is valid.

Thus the sum of solutions is

1+(1)=01 + (-1) = 0

Therefore, the correct option is B.

Use reciprocal-log substitution

Given: log(x+3)(6x2+28x+30)=52log(6x+10)(x2+6x+9)\log_{(x+3)}(6x^2 + 28x + 30) = 5 - 2\log_{(6x+10)}(x^2 + 6x + 9)

Find: The sum of all real solutions.

After factoring,

6x2+28x+30=(x+3)(6x+10),x2+6x+9=(x+3)26x^2+28x+30 = (x+3)(6x+10), \qquad x^2+6x+9 = (x+3)^2

so the equation reduces to expressions involving

log(x+3)(6x+10)\log_{(x+3)}(6x+10)

and its reciprocal. This works because

logab=1logba\log_a b = \frac{1}{\log_b a}

Let

y=log(x+3)(6x+10)y = \log_{(x+3)}(6x+10)

Then the equation directly becomes

1+y=54y1+y = 5 - \frac{4}{y}

which gives

(y2)2=0(y-2)^2 = 0

Hence,

y=2y=2

Now use the meaning of the logarithm:

log(x+3)(6x+10)=26x+10=(x+3)2\log_{(x+3)}(6x+10)=2 \Rightarrow 6x+10=(x+3)^2

So,

x2=1x^2=1

and the real solutions are x=1x=1 and x=1x=-1. Their sum is 00. Therefore, the correct option is B.

Common mistakes

  • A common mistake is to miss the factorization 6x2+28x+30=(x+3)(6x+10)6x^2+28x+30 = (x+3)(6x+10) and x2+6x+9=(x+3)2x^2+6x+9 = (x+3)^2. Without this simplification, the log expressions do not reduce neatly. Always factor first before applying logarithmic properties.

  • Students often forget the reciprocal identity logab=1logba\log_a b = \frac{1}{\log_b a} and incorrectly treat log(6x+10)(x+3)\log_{(6x+10)}(x+3) as equal to log(x+3)(6x+10)\log_{(x+3)}(6x+10). They are reciprocals, not equal in general.

  • Another mistake is to solve for xx from x2=1x^2=1 and stop there without checking the logarithm domain. For logarithms, the bases must be positive and not equal to 11, and the arguments must be positive. Always verify each obtained root in the original equation.

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