Question

if a^2+b^2+c^2 =90 and a+b+c= 20 what is ab+bc+cd?

Answer 1

(a + b + c)2 = a2 + b2 + c2 + 2ab + 2bc + 2ca
(a+ b + c)2 = a2 + b2 + c2+ 2 (ab + bc + ca)
(20)2 = 90 + 2 (ab + bc + ca)
400 - 90 = 2 (ab + bc + ca)

ab + bc + ca = 155




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Answer 2

Answer:

155 is the answer . by using (a+b+c)^2