y=3cos(2x-π/6)
2kπ-π < 2x-π/6 < 2kπ,
即kπ-5π/12 < x < kπ+π/12 (k∈Z)时,单调增
2kπ < 2x-π/6 < 2kπ+π,
即kπ+π/12 < x < kπ+7π/12(k∈Z)时,单调减
y=3cos(2x-π/6)
2kπ-π < 2x-π/6 < 2kπ,
即kπ-5π/12 < x < kπ+π/12 (k∈Z)时,单调增
2kπ < 2x-π/6 < 2kπ+π,
即kπ+π/12 < x < kπ+7π/12(k∈Z)时,单调减