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(Exterior Angle Theorem): If a side of a...

(Exterior Angle Theorem): If a side of a triangle is produced, the exterior angle so formed is equal to the sum of the two interior opposite angles. GIVEN : A triangle `A B CdotD` is a point of `B C` produced, forming exterior angle `/_4.` TO PROVE : `/_4=/_1+/_2` i.e. , `/_A C D=/_C A B+/_C B Adot`

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`In /_ABC`
`/_1+/_2+/_3=180^@-(1)`
`/_3+/_4=180^@-(2)`
from equation 1 and 2
`/_1+/_2+/_3=/_3+/_4`
`/_1+/_2=/_4`
`/_CAB+/_CBA=/_ACD`.
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