(a²+9)²-36a²
=a^4+18a²+9²-36a²
=a^4-18a²+9²
=(a²-9)²
=(a+3)²(a-3)²（x²+5x）²+8x²+40x+16呢（x²+5x）²+8x²+40x+16 =（x²+5x）²+8(x²+5x)+16 =（x²+5x+4）² =(x+1)²(x+4)²