Question

Use cramers rule to solve each system 6x +2y= -78 7x - 4y= -15

Answer 1

`\bf \begin{cases} 6x+2y=-78\\ 7x-4y=-15 \end{cases}\qquad \begin{bmatrix} 6&2\\7&-4 \end{bmatrix}\implies (6\cdot -4)-(7\cdot 2)\implies \stackrel{D}{\boxed{-38}}\\\\ -------------------------------\\\\ x=\cfrac{D_x}{D}\implies \cfrac{ \begin{bmatrix} -78&2\\-15&-4 \end{bmatrix}}{-38}\implies \cfrac{(-78\cdot -14)-(-15\cdot 2)}{-38} \\\\\\ \cfrac{312+30}{-38}\implies\cfrac{342}{-38} \implies -9\\\\ -------------------------------`


`\bf y=\cfrac{D_y}{D}\implies \cfrac{ \begin{bmatrix} 6&-78\\7&-15 \end{bmatrix}}{-38}\implies \cfrac{(6\cdot -15)-(-7\cdot 78)}{-38} \\\\\\ \cfrac{-80+546}{-38}\implies \cfrac{456}{-38}\implies -12`