Chọn C
Ta có:
\(\int _{-1}^{2}f(t)dt= \int _{-1}^{2}f(x)dx= 9. \)
Áp dụng công thức: \(\int _{a}^{c}f(x)dx +\int _{c}^{b}f(x)dx=\int _{a}^{b}f(x)dx \, \, \, \, \, \, \, \left(a<c<b\right). \)
\(\int _{-1}^{2}f(x)dx =\int _{-1}^{1}f(x)dx +\int _{1}^{2}f(x)dx \Rightarrow \int _{1}^{2}f(x)dx =\int _{-1}^{2}f(x)dx -\int _{-1}^{1}f(x)dx =9-7=2.\)