\left \begin{array} { l } { \text { (4) } ( a \cos \alpha . 2 a \cos \alpha ) } \\ { \text { f } y + b = m _ { 1 } ( x + a ) \text { and } y + b = m _ { 2 } ( x + a ) \text { are } } \\ { \text { wo tangents to the parabola } y ^ { 2 } = 4 a x . \text { then } } \\ { \left \begin{array} { l l l l } { \text { (1) } m _ { 1 } + m _ { 2 } = 0 } & { \text { (2) } m _ { 1 } m _ { 2 } = 1 } \end{array} \right } \end{array} \right