x²-7x+12 = 0
x1² -7x1 +12 = 0 |:25
x2² -7x2 +12 = 0 |:25
=> x1²/25 -7x1/25+12/25= 0
=> x2²/25 -7x2/25+12/25 = 0
=> (x1²/25 + x2²/25) - 7/25 • (x1+x2)+ 6/25 = 0
=> (x1²/25 + x2²/25) -7/25 •7 + 6/25 = 0
=> x1²/25 + x2²/25 = (49+6)/25
=> x1²/25 + x2²/25 = 55/25
=> (x1²/25 + x2²/25) = 11/5
=> x1²+x2² = 55
b)E = ((x1+x2)³ - x1³-x2³)/(x1x2•(x1+x2))
= ((7³-(x1³+x2³))/(12•7)
x²-7x+12 = 0 |•x
x³-7x²+12x = 0
=> x1³+x2³-7(x1²+x2²)+12(x1+x2) = 0
=> x1³+x2³-7•55+12•7 = 0
=> x1³+x2³ = 301
=> E = (343-301)/84 = 42/84 = 1/2