22.(本小题10分)如图,约定:上方相邻两整式之和等于这两个整式下方箭头共同指向的整式.
(1)求整式M,P.
(2)将整式P因式分解.
(3)P的最小值为.

22.【解】(1)根据题意,得$M = (3x^2 - 4x - 20) - 3x(x - 3) = 3x^2 - 4x - 20 - 3x^2 + 9x = 5x - 20$.$P = 3x^2 - 4x - 20 + (x + 2)^2 = 3x^2 - 4x - 20 + x^2 + 4x + 4 = 4x^2 - 16$.(2)$P = 4x^2 - 16 = 4(x^2 - 4) = 4(x + 2)(x - 2)$.(3)$-16$ 分析:$\because P = 4x^2 - 16$,$x^2 \ge 0$,$\therefore$当$x=0$时,$P$有最小值,最小值为$-16$.