# Homework Help: [Linear Algebra] Showing equality via determinant properties

1. Sep 15, 2010

### CentreShifter

Problem:

Show, without evaluating directly, that
$$\left|\begin{matrix} a_1+b_1t&a_2+b_2t&a_3+b_3t \\ a_1t+b_1&a_2t+b_2&a_3t+b_3 \\ c_1&c_2&c_3 \end{matrix}\right| = (1-t^2)\left|\begin{matrix} a_1&a_2&a_3 \\ b_1&b_2&b_3 \\ c_1&c_2&c_3 \end{matrix}\right|$$

Clearly, here I'm supposed to use the determinant properties, do some row ops on the first array, and end up with the RHS.

1. -tR1-R2 -> Row2 (no coefficient on determinant).

$$\left|\begin{matrix} a_1+b_1t&a_2+b_2t&a_3+b_3t \\ b_1-b_1t^2&b_2-b_2t^2&b_3-b_3t^2 \\ c_1&c_2&c_3 \end{matrix}\right|$$

From here I can see that $$b_1-b_1t^2=b_1(1-t^2)$$. But multiplying R2 by $$\frac{1}{1-t^2}$$ means I have to also pull that out as a coefficient to the entire array. So now:
$$\frac{1}{1-t^2} \left|\begin{matrix} a_1+b_1t&a_2+b_2t&a_3+b_3t \\ b_1&b_2&b_3 \\ c_1&c_2&c_3 \end{matrix}\right|$$

Another row op, R1 -> R1-tR2, and I have my RHS, except the coefficient is reciprocated. Am I doing this wrong?

2. Sep 15, 2010

### ehild

If you multiply a row or column by some t, the value of the determinant becomes t times the original value. So you need to divide the whole determinant by t so as to keep it unchanged.

ehild