Question

If a + b + c = 11 and a2 + b2 + C2 = 81, find :
ab + bc + ca.​

Answer 1

Answer:

ab +bc +ca is 20 hence 20 is correct answer

Answer 2

Given

a + b + c = 11

a^2 + b^2 + c^2 = 81

Find

ab + bc + ca

solution

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

=> (11)^2 = 81 + 2 (ab + bc + ca)

=> 2 (ab + bc + ca) = 121 - 81

=> 2( ab + bc + ca) = 40

=> (ab + bc + ca) = 40 / 2

=> ab + bc + ca = 20