In a certain clothing store 6 shirts and 3 ties cost $79.50 and 3 shirts and 2 ties cost $41 determine the cost of each shirt

9514 1404 393

  $12 for each shirt

Step-by-step explanation:

If we let 's' and 't' represent the costs of a shirt and tie, respectively, then we can write the general form equations ...

  6s +3t -79.50 = 0

  3s +2t -41.00 = 0

The solution for s can be found using the "cross multiplication" technique.

Differences of products can be formed:

  d1 = (6)(2) -(3)(3) = 3

  d2 = (3)(-41) -(2)(-79.50) = 36

Then we have ...

  1/d1 = s/d2   ⇒   s = d2/d1 = 36/3 = 12

The cost of each shirt is $12.00.

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2 ties cost 41-36 = 5, so each one is $2.50.

Additional comment

The use of simple elimination on this set of equations would eliminate the s-variable, so would mean additional work to find s after finding t. The t-variable could be eliminated by multiplying one equation by one coefficient, and the other by a different coefficient. As long as we're doing that amount of work, we may as well do just enough cross-multiplication to get the one answer we need. This is where the "cross-multiplication" method of solution comes in handy.

This method solves the general-form equations ...

by considering the two rows of coefficients:

Then differences of cross-products are formed in adjacent columns:

  d1 = ae -db

  d2 = bg -ec

  d3 = cd -ga

The solutions are ...

  1/d1 = x/d2 = y/d3   ⇒   x = d2/d1, y = d3/d1

3. parallelogram

step-by-step explanation:

we turn each of our sides into direction vectors

ab=b-a=(3 - 7, 1 - 0) = (-4, 1)

bc=c-b=(1,6)

cd=d-c=(4,-1)

da=a-d=(-1,-6)

we see how the opposite sides have negated direction vectors; this is characteristic of a parallelogram.   we have congruent sides ab=cd and bc=da with unequal lengths because 1^2+6^2 is different from (-4)^2+1^2.   so it's not a square or a rhombus.  

we check the dot product of direction vectors of adjacent sides.   we have

ab dot bc = -4(1) + 1(6) = 2

that's not zero so the sides are not perpendicular.   we've ruled out rectangle.

the direction vectors indicate two pairs of congruent parallel sides, so we choose parallelogram over trapezoid.