I can’t understand why everyone complicates this question. It can be easily solved by similar triangles.
In this png, we have something to make sure.
∠B=∠DAB
∠
B
=
∠
D
A
B
(Yes, dab)
This also means AD=BD
A
D
=
B
D
.
This is our basic construction of D, which is going to help us.
∠ADC=∠DAB+∠B=2∠B=∠CAB
∠
A
D
C
=
∠
D
A
B
+
∠
B
=
2
∠
B
=
∠
C
A
B
∠CAD=∠CAB−∠DAB=∠B
∠
C
A
D
=
∠
C
A
B
−
∠
D
A
B
=
∠
B
These are based on the fact that ∠A=2×∠B
∠
A
=
2
×
∠
B
Actually these conditions suffice. Because I am just proving that △ACD∼△BCA
△
A
C
D
∼
△
B
C
A
Similarity makes us realize the following:
ACBC=ADAB
A
C
B
C
=
A
D
A
B
and
ACBC=CDAC
A
C
B
C
=
C
D
A
C
So
AC×AB=BC×AD
A
C
×
A
B
=
B
C
×
A
D
and
AC2=BC×CD
A
C
2
=
B
C
×
C
D
So
BC2=BC×(BD+CD)=BC×(AD+CD)
B
C
2
=
B
C
×
(
B
D
+
C
D
)
=
B
C
×
(
A
D
+
C
D
)
=AC×AB+AC2
=
A
C
×
A
B
+
A
C
2
Q.E.D.
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Answer:
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We are given:
- Parallelogram CDEF
- ∠C = 13x - 10
- ∠D = 5x + 10
To Find: ∠E
Concept Used:
- Adjacent angles of a Parallelogram are Supplementary
Finding the value of 'x':
Since adjacent angles of a parallelogram are Supplementary:
∠D + ∠C = 180
5x + 10 + 13x - 10 = 180 [replacing the values of angle C and D]
18x = 180
x = 10 [dividing both sides by 18]
__________________________________________________________
Finding Measure of Angle D:
We are given that:
∠D = 5x + 10
∠D = 5(10) + 10 [since x = 10]
∠D = 60°
__________________________________________________________
Finding the measure of ∠E:
∠D and ∠E are Adjacent angles
Since Adjacent angles are Supplementary:
∠D + ∠E = 180
60 + ∠E = 180
∠E = 120° [subtracting 60 from both sides]
Hence, the measure of ∠E = 120°
Answer:
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