点(1,2)(1,2)(1,2)を通るので 2=1+a+b2 = 1 + a + b2=1+a+b a+b=1①a + b = 1 \quad ①a+b=1① y′=3x2+2ax+by' = 3x^2 + 2ax + by′=3x2+2ax+b (1,2)(1,2)(1,2)で傾き−1-1−1なので −1=3+2a+b-1 = 3 + 2a + b−1=3+2a+b 2a+b=−4②2a + b = -4 \quad ②2a+b=−4② ①、②より a=−5a = -5a=−5 b=6b = 6b=6