Chapter 6: Tales by Dots and LinesClass 8 Mathematics — summary, notes, extra questions & MCQ quiz

Summary

This data-handling chapter deepens the ideas of mean and median and introduces line graphs. The mean is reinterpreted as a balance point: the total distance of values to its left equals the total distance to its right, and there is only one such centre. Including a value above the mean raises it, and below it lowers it; the same logic governs the median, the middle value of sorted data. Algebra shows that adding a fixed number \(k\) to every value raises the mean by \(k\), and multiplying every value by \(k\) multiplies the mean by \(k\). The mean and median can be found efficiently from frequency tables. Spreadsheets are introduced as a tool, with formulas like \(\text{SUM}\) and \(\text{AVERAGE}\) to compute totals and averages quickly. Line graphs visualise change over time — temperature, rainfall, space launches and sleep durations — and the chapter stresses reading a graph in two steps: first identify what is given (scale, axes, marks), then infer and interpret trends, comparing steepness to judge rates of change.

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