if a+b=6 and a^2+b^2=20,Then ab=?
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b=6-a
a²+b²=20
a²+b²+2ab=20
a²+(6-a)²+2ab=20
a²+36+a²-12a=20
2a²-12a+16=0
2(a²-6a+8)=0
a²-6a+8=0
(x-4)(x-2)=0
x=4,2
when a=4
b=6-4=2
when a=2
b=6-2=4
ab=1 st case=4×2=8
ab=2nd case=2×4=8
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