Question

None
11. If x + y + z = 13 and x2 + y2 +
z2 = 69, then xy + z (x + y) is
equal to....... the answer​

Answer 1

Answer:

xy + z(x+y) = xy+yz+zx

x+y+z = 13

=> (x+y+z)² = 169

=> x²+y²+z² + 2(xy+yz+zx) = 169

=> 69 + 2(xy+yz+zx) = 169

=> 2(xy+yz+zx) = 100

=> xy + yz + zx = 50

=> xy + z(x+y) = 50