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Question

If a + b + c = 13 and ab + bc + ca = 4, then what is the value of ab(a + b) + bc(b + c) + ca(c + a) + 3abc?

The correct answer is
52

To solve this problem, we need to find the value of the expression:

\(ab(a + b) + bc(b + c) + ca(c + a) + 3abc\)

given the conditions:

  • \(a + b + c = 13\)
  • \(ab + bc + ca = 4\)

We can start by expanding the given expression:

\(ab(a + b) + bc(b + c) + ca(c + a) + 3abc = a^2b + ab^2 + b^2c + bc^2 + c^2a + ca^2 + 3abc\)

We can rewrite the expression as:

\(a^2b + ab^2 + b^2c + bc^2 + c^2a + ca^2 + 3abc = a^2b + ab^2 + b^2c + bc^2 + c^2a + ca^2 + abc + 2abc\)

Notice the terms can be rearranged into \(ab(a + b) + bc(b + c) + ca(c + a) = abc + (a^2b + ab^2 + b^2c + bc^2 + c^2a + ca^2 + 2abc)\).

By substituting \(a + b + c\) and \(ab + bc + ca\), we can solve:

\((a+b+c)^2 = a^2 + b^2 + c^2 + 2(ab + bc + ca)\)

We already know \(a + b + c = 13\) and \(ab + bc + ca = 4\). Using these values, calculate:

\(169 = a^2 + b^2 + c^2 + 8\)

Thus, \(a^2 + b^2 + c^2 = 161\).

Now rearrange \(a^2 + b^2 + c^2\) in terms and plug it back:

\(a^2b + ab^2 + b^2c + bc^2 + c^2a + ca^2\) forms part of \((ab + bc + ca)(a + b + c) - 3abc\).

Calculate \((ab + bc + ca)(a + b + c)\):

\(= (4)(13) = 52\)

Therefore, expression simplifies as:

\(ab(a + b) + bc(b + c) + ca(c + a) + 3abc = 52 - 3abc + 3abc = 52\)

Therefore, the value of the given expression is 52.

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Important Questions from Algebra (Notes)

  1. If $(y-12) = 4\sqrt{5}$, then find the value of $\sqrt{y-3} - \frac{1}{\sqrt{y-3}}$.
  2. In the expansion of (x + 9)(x - 6)(x + 5), what is the coefficient of x?
  3. The roots of the equation $ax^3-24x^2+188x-480=0$ are three consecutive even natural numbers. The value of a is _____.
  4. A square matrix having all the elements above the leading diagonal equal to zero is known as:
  5. The difference between two numbers is 16. If one-third of the smaller number is greater than one-seventh of the larger number by 4, then what is the larger number?
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