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a) the property of the inscribed quadrilateral is: m∠A+m∠G=m∠H+m∠F=180°.
It means (for m∠A+m∠G=180°), 21x-2+38x+5=180°, ⇒ x=3, m∠G=63-2=61°.
b) m(arcFAH)=2m∠FGH=2m∠G=122°.
c) x²-2x+y²-4y=11; ⇔ (x²-2x+1)+(y²-4y+4)=16; ⇔ (x-1)²+(y-2)²=4², where (1;2) is the centre and 4 - radius of this circle.