If a + b + c = 13, a2 + b2 + c2 = 69 then find the ab + bc + ca : Pick
A) 10  B) 30  C) 50  D) 70  E) 90
Explanation:   (a + b + c)2 = a2 + b2 + c2 + 2(ab + bc + ca)   2(ab + bc + ca) = (a + b + c)2 - a2 + b2 + c2   2(ab+bc+ca) = 169 - 69= 
<gadget id="calculator">169 - 69</gadget>
<output>100</output>
100.   (ab + bc + ca)= 50.   Answer: 50
<result>C</result>
If a + b + c = 13, a2 + b2 + c2 = 69 then find the ab + bc + ca :