Is the expression 5x²(2/5-x) a trinomial?

Of the ones listed here, the factor that had the greatest impact on the eventual destruction of native americans' ways of life was the development of the a) automobile. b) highway. c) railroad. d) steamboat.

Which expressions are equivalent to k/2 choose 2 answers: a k-2 b 2/k c 1/2k d k/2 e k+k

A. 127 cm^2 b. 144.5 cm^2 c. 172 cm^2 d. 50 cm^2