Question

if x^2+y^2+z^2=40 and xy+yz+zx=30. find x+y+z.
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Answer 1

(x+y+z)^2=x^2+ y^2+ z^2+2(xy+yz+zx)
=40+2×30
=100

Answer 2

(x+y+z)² = x²+y²+z²+2xy+2yz+2zx

(x+y+z)² = x²+y²+z²+2(XY+yz+zx)

(x+y+z)² = 40+2(30)

(x+y+z)² = 40+60 = 100

(x+y+z) = 10

Formula used,

(a+b+c)² = a²+b²+c²+2ab+2bc+2ca