b^2-4b-21=0

解

b^{2} - 4 b - 21 = 0

b = 7, b = - 3

解答ステップ

b^{2} - 4 b - 21 = 0

解くとthe二次式

b_{1, 2} = \frac{- ( - 4 ) \pm ( - 4 ) ^{2} - 4 \cdot 1 \cdot ( - 21 )}{2 \cdot 1}

(- 4)^{2} - 4 \cdot 1 \cdot (- 21) = 10

b_{1, 2} = \frac{- ( - 4 ) \pm 10}{2 \cdot 1}

解を分離する

b_{1} = \frac{- ( - 4 ) + 10}{2 \cdot 1}, b_{2} = \frac{- ( - 4 ) - 10}{2 \cdot 1}

b = \frac{- ( - 4 ) + 10}{2 \cdot 1} : 7

b = \frac{- ( - 4 ) - 10}{2 \cdot 1} : - 3

二次equationの解:

b = 7, b = - 3