Observe the figures, Then the triangles are right angular triangles.

Let the first second triangles are , and .

The sum of the measures of the angles in a triangle is 180°.

The right triangle measures ∠C = ∠F = 90°, and ∠A = 55°.

In , Let x = the measure of ∠B.

∠A + ∠B + ∠C = 180°

55° + x + 90° = 180°                    (Substitute ∠A = 55° and ∠C = 90°)

x + 145° = 180°                            (Add: 90° + 55° = 145°)

Apply subtraction property of equality: If a = b then .

x + 145° 145° = 180° 145° (Subtract 145° from each side)

x = 180° 145°                           (Apply additive inverse property: 145° 145° = 0)

x = 35°                                          (Add: 180° 145° = 35°)

So ∠B = ∠E = 35°.

Since the corresponding angles are equal measures, .

The corresponding angles are equal measures, .