a + b + c = 15 এবং a2 + b2 + c2 = 83 হলে ab + bc + ca = কত ?
Correct Answer: Option C
a+b+c = 15 , a2 +b2 +c2 = 83,
We know that,
2(ab+bc+ca ) = (a+b+c )2 - (a2 +b2 +c2 )
or, ab+bc+ca = \(\frac{{(a + b + c)2 - (a2 + b2 + c2)}}{2} = \frac{{15 - 83}}{2} = \frac{{225 - 83}}{2}\) = 71
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