Let f(x) = ax3 + bx2 + cx + 41 be such that f(1) = 40, f'(1) = 2 and f''(1) = 4. Then a2 + b2 + c2 is equal to:
- 54
- 51
- 73
- 62
Answer
(2) 51
Solution
f(1) = 40 a + b + c = -1 …(i)
f'(x) = 3ax2 + 2bx + c 3a + 2b + c = 2 …(ii)
f''(x) = 6ax + 2b 6a + 2b = 4 3a + b = 2 …(iii)
(ii) − (iii) gives b + c = 0, so from (i) a = -1; then (iii) gives b = 5 and c = -5
a2 + b2 + c2 = 1 + 25 + 25 = 51
Answer: (2)