In the following questions, what is the value of z_1.z_2 ?
\left(3-i\right)\times\left(2+5i\right)
\left(1-2i\right) \times \left(1 + 2i\right)
\left(1-4i\right) \times \left(2 + i\right)
\left(2-3i\right) \times \left(5 - 6i\right)
\left(5+7i\right) \times \left(6 - 2i\right)
\left(3-9i\right) \times \left(2 - 8i\right)
\left(3-7i\right) \times \left(3 +7i\right)