化简求值(28a3-28a2+7a)/7a,a=3/4

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化简求值(28a3-28a2+7a)/7a,a=3/4
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化简求值(28a3-28a2+7a)/7a,a=3/4
化简求值(28a3-28a2+7a)/7a,a=3/4

化简求值(28a3-28a2+7a)/7a,a=3/4
原式=28a3/7a-28a2/7a+7a/7a
=4a2-4a+1
=4×9/16-4×3/4+1
=1/4

tana*cota=1
(tan a)^2+(cot a)^2
=(tana+cot)^2-2tana*cota
=2^2-2
=2
所以(tan a)^3+(cot a)^3
=(tana+cota)[(tan a)^2-tana*cota+(cot a)^2]
=2[(tan a)^2+(cot a)^2-1]
=2*(2-1)
=2