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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. A square matrix having all the elements above the leading diagonal equal to zero is known as:
  2. If $(\frac{7}{11})^{k-5} = (\frac{11}{7})^{k-9}$, find the value of $2^k$.
  3. The sum of the age of A and 5 times the age of B is 36 years. When 3 times the age of A is added to 7 times the age of B, the result is 62 years. The sum of the ages (in years) of A and B is:
  4. 18 bags and 3 pens together cost ₹1,164, whereas 14 bags and 2 pens together cost ₹922. The cost of 9 bags exceeds the cost of 2 pens by:
  5. The cost of 2 pens and 5 notebooks is ₹187. The cost of 8 notebooks exceeds the cost of 4 pens by 40. What is the cost of 8 pens and 7 notebooks?
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