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Ninth grade mathematical equation
△=(2k+ 1)? -4(k? +k)

=4k? +4k+ 1-4k? -4k

= 1 & gt; 0

∴AB is not equal to AC.

When △ABC is an isosceles triangle, one side =BC=5.

∴ Substitution x=5

5? -5(2k+ 1)+k? +k=0

k? -9k+20=0

K=5 or k=4

( 1)k=5,x? - 1 1x+30=0

∴x=6, x=6, namely AB=5, AC=6.

∴△abc perimeter =5+5+6= 16

Area = 1/2×6×4= 12.

(2)k=4,x? -9x+20=0

x=5,x=4

∴△ABC perimeter =4+5+5= 14

Area =1/2× 4×√ 21= 2 √ 21.