= C₈⁴ = 8*7*6*5/(1*2*3*4) = 70
f'(x) = 18x² - 10x + 4,恒为正,即f(x)为单调递增
f(0) = -15 < 0
f(1) = -10 < 0
f(2) = 21 > 0
有一个根在1,2之间,容易试出是x = 3/2
f(x) = (2x + 3)(3x² + 2x + 5)
显然后者无实数解,即f(x) = 0只有一个实数解
f'(x) = -21x^6 + 10x⁴ - 3
f"(x) = -126x^5 + 40x^3 = 2x^3(20 - 63x²
易证x² = 20/63时,f'(x)取最大值,且此最大值小于0,f(x)单调递减增