Question

Cho x + y = 5. Tinh GT bieu thuc:

\(P=3x^2-2x+3y^2-2y+6xy-10\)

Answer 1

P = 3x² - 2x + 3y² - 2y + 6xy - 10 = (3x² + 6xy + 3y²) - (2x + 2y + 10) = 3(x² + 2xy + y²) - 2(x + y + 5) 

   = 3(x + y)² - 2.(5 + 5) = 3.5² - 2.10 = 75 - 20 = 55

Answer 2

\(P=3x^2-2x+3y^2-2y+6xy-10\) 

\(P=3\left(x^2+y^2\right)-2\left(x+y\right)+6xy-10\)

\(P=3\left[\left(x+y\right)-2xy\right]-10+6xy-10\)

\(P=3\left(5-2xy\right)-20+6xy\)

\(P=15-6xy-20+6xy\)

\(P=15-20=-5\)

Answer 3

P= ( 3x² + 6xy + 3y² ) -2x - 2y -10

  = 3.x² +2xy + y² ) -2( x+y) -10 

  = 3.(x+y)²  -2(x+y) -10 

  = 3.5 -2.5 -10 

  = 15-10-10 

  = -5