Given 8x-3y=-21, 3x+y=-10; Prove x=-3
1 answer:
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Consider two triangles AEB and CEF. In these triangles:
- ∠AEB≅∠CEF (as vertical angles);
- ∠EAB≅∠ECF (lines AB and CD are parallel and AC is transversal, then these angles are congruent as alternate interior angles);
- ∠ABE≅∠CFE (lines AB and CD are parallel, BF is transversal, then these angles congruent as alternate interior angles).
Thus, by AAA theorem,
Corresponding sides of similar triangles are proportional, then
ABCD is a parallelogram, then AB=CD.
Therefore,
CD=2CF and CD=DF+CF. Equate these two expressions:
DF+CF=2CF,
DF=CF.
This gives you that DF:CF=1:1.
Answer: 1:1